Category Archives: knowledge

subtractive mixing – why not RGB?

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In a previous post I spoke about the difference between additive and subtractive mixing and why the additive primaries are red, green and blue or RGB for short – http://colourware.wordpress.com/2009/07/13/additive-colour-mixing/

The chromaticity diagram – see http://colourware.wordpress.com/2009/09/28/colourchat-audiovisual-guide-to-the-chromaticity-diagram/ – has a very useful property. If you plot the chromaticities of two lights, then the straight line that joins the two points on the chromaticity diagram show you the additive mixtures that can be obtained by mixing together the two lights. If we take three lights, then the additive mixtures that can be obtained are defined by the triangle that is formed if the chromaticities are the vertices of the triangle. Ok – that’s a bit of a mouthful so let’s have a practical example. The triangle in the diagram below shows the gamut that can be achieved when we have three additive primaries that are positioned at the corners of the triangle.

rgb_gamut

 From this diagram it should become obvious why the additive primaries are RGB. Say, we chose, two reds and a cyan as the three additive primaries – well, the triangle would be tiny. In other words, the gamut would not be very big. The biggest triangle in the chromaticity is one whose vertices are formed by a red, a green and a blue. WhichRGB will give the biggest triangle? I don’t know – it’s been something that has been puzzling me for the last few days and I’ll come back to this in a later post. But certainly any RGB triangle is pretty large as long as the red, green and blue primaries chosen are reasonably saturated.

So what happens if we choose RGB as the subtractive primaries? Subtractive colour mixing describes how inks and paints mix together to form colours. The first thing to point out is that subtractive colour mixing is not additive and linear – you remember I said that when you mix two lights together the colour mixtures all fall on the straight line that joins the  two points in the chromaticity diagram that represent the two lights? Well, this is only true for additive colour mixing. So to work out the gamut for subtractive systems is not an easy thing to do. However, if you do select the three subtractive primaries as RGB you’ll get a gamut that looks something like this:

rgb_subgamut

Notice that the gamut is concave. Mixing red and green lights produces a nice yellow. You can test this by going into your colour-picker in software such as Photoshop or Powerpoint and setting the RGB values to be 255:255:0. You’ll get a nice yellow. But mixing red and green paints – it will give you a similar hue to yellow but you’ll get something quite desaurated; most likely you’ll get a brown. So using RGB as the subtractive primaries would not be a very good thing at all.

It turns out that additive and subtractive colour mixing are very related. The best subtractive primaries are the ones that control the amount of red, green and blue light reflected. A yellow dye applied to textiles, for example, mainly absorbs short wavelengths in the blue section of the spectrum, allowing the other wavelengths to be reflected by the textile. The “other wavelengths” that are reflected give yellow. But the important point is that the yellow dye absorbs blue. Similarly, a magenta dye absorbs green and a cyan dye absorbs red. This leads to the idea of the optimal subtractive primaries being those that are cyan, magenta and yellow or CMY. This leads to a gamut somewhat like this:

cmy_gamut

The biggest gamut for subtractive mixing is obtained by using CMY as the primaries. But weren’t you taught at school that the subtractive primaries are red, blue and yellow? Almost certainly you were – and this is because it is accepted dogma at most art colleges and in many art and design textbooks. But it is quite easy to show that the optimal primaries – those giving the largest gamut – are CMY not RBY. If you were building a colour-reproduction system using only three colours such as a printer you would come to the conclusion – as companies such as HP, Xerox, and Epson have done – that you get the largest colour range with CMY. So why has it become commonplace for artists to refer to red, yellow and blue as the primaries? Could it be a colour naming and language issue – that they really mean cyan when they say blue and it’s just a naming error. Possible, but not likely in my opinion.  I think it is more likely that most artists are not overly concerned that RYB gives a smaller gamut than CMY because they rarely restrict themselves to three primaries. An artist would typically use 6 or more primaries. For example, they might use two blues (one that is reddish and one that is greenish), two reds (one that is yellowish and one that is bluish) and two yellows (one that is greenish and one that is reddish) in order to easily be able to mix a wide range of colours. The (mis-)identification of RYB as the subtractive primaries has much to do with colour wheels. I like to keep each of these blog posts reasonably concise – if I start writing about the problems of colour wheels now I will be writing for another 2 hours. And it’s nearly midnight now so colour wheels will need to wait for another day!

red-blue flicker causes epilepsy

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In 1997, more than seven hundred children in Japan suffered an epileptic attack while watching an episode of Pokemon cartoon. This was later diagnosed as a case of photosensitive epilepsy (a kind of epilepsy caused by visual stimulus) triggered by a specific segment of the cartoon containing a colourful flickering stimulus. However, despite the ubiquitous presence of colourful displays and materials, very little is known about the relationship between colour-combinations (chromaticity) and photosensitivity. A new study has recently been published that suggests that certain colour combinations, for example, red-blue flickering stimuli, cause larger cortical excitation than other colour combinations such red-green or blue-green stimuli – http://www.goldsmiths.ac.uk/press-releases/pressrelease.php?releaseID=749

What is a colour space?

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In my job I probably use the phrase “colour space” every day and have done for the last 20 years. So imagine my surprise when I was talking with a colleague recently and after a few minutes he said “Can I stop you for a second there Steve – when you say colour space, what exactly do you mean?”.

A colour space is like a map. A map of New York, for example, shows the location of various landmarks with reference to the xy coordinates (the position in horizontal x and vertical y units on the map). A colour space or colour map does the same thing with colours. Perhaps the simplest colour space is the spectrum, see below:
 myspectrum

As we look from right to left on the spectrum the wavelengths changes from around 700nm on the far left to about 400nm on the far right. So this map shows colour with reference to wavelength. Although it is a commonly used colour space it is limited because it only really describes how hue changes with wavelength. Hue is only one of three ways in which colour can change or vary.

The most well-known really useful colour space then is the CIE chromaticity diagram – see below.

chromdiagram

The CIE chromaticity diagram shows colours arranged on a 2-D plane. We can easily refer to any colour by how far from the left it is (the x coordinate) and how far from the bottom it is (the y coordinate). This space only shows two of the dimensions of colour; the hues are arranged in a somewhat circular way and the colourfulness increases as we move outwards from the white point (a position near to the centre of the diagram). However, we can also consider the third component of colour (brightness) if we imagine a dimension coming out of the page towards you (http://colourware.wordpress.com/2009/07/18/cie-system-of-colorimetry/). The CIE defines several different colour spaces; the CIELAB colour space, for example, is another 3-D space that defines a colour by its L*, a* and b* values.

It is useful to think of an image-display device as also having a colour space. Consider the display on which you are probably reading this blog. The display shows colour by changing the amount of the red, green and blue light emitted at each point on the screen. The diagram below is a representation of what the RGB colour space of your display device may look like.

 

rgb

In the RGB cube, black is in the bottom left. As the RGB values increase colours are created and white results from each of the RGB primaries at full strength. So the RGB colour space defines the relationship between RGB values and colour. However, here’s the really interesting thing: The colour space for different display devices is very different. Even if we take a single device – such as the one that you are reading this blog on – then as we change settings (the brightness, the contrast, the gamma, the colour temperature, etc.) then the colour space changes. That is, the relationship between RGB and colour changes as you change those settings. This is a huge problem. Imagine if there were many maps of New York and each showed the position of, say, the Empire State Building to be in a different position. How confusing would that be? Well, that’s the problem with colour-display technology. If we didn’t do anything about this problem then every time we looked at a colour image on a different display device the colours could change markedly. This is why we need colour management. Colour management can make compensations to the RGB values that are sent to each display device so that the colours always appear the same (well, nearly the same). To make this compensation the colour management software (which is embedded in your Windows or Apple operating system) needs to know about the colour space of each device connected to the computer. Each device needs to have a profile that describes the relationship of its own colour space with respect to some standard colour space. 

How good is colour management? Well, that depends upon many factors. Most printers, cameras, scanners, and screens (LCD, CRT, etc.) come with a driver that includes a crude colour profile. This ensures that there is a basic level of colour management and for a great majority of users this is more than adequate. However, if you want better performance then you need to think about making some measurements that will allow a more accurate colour profile to be built. In a recent blog I described a new device that you can buy to enable you to do this – http://colourware.wordpress.com/2009/07/29/colormunki-colour-management/. There are many such devices on the market. I highly recommend Andrew Rodney’s book Color Management for Photographers which is both clear and accurate (though the edition I have works on Adobe’s CS2 package whereas the latest package is CS4).

514PWPFTKXL__BO2,204,203,200_PIsitb-sticker-arrow-click,TopRight,35,-76_AA240_SH20_OU02_

However, no matter how hard you try, colour management is never likely to be perfect. This is because different devices have different colour gamuts; a printer is likely to be able to display some colours that your display physically cannot and vice versa.

Indigo – a colour of the rainbow?

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From time to time I come across web pages and groups of people who get irrate about indigo being in the rainbow. There is even a facebook group called “Get Indigo out of the rainbow”. It was Newton who suggested that the rainbow contains seven colours: red, orange, yellow, green, blue, indigo and violet. It has been suggested that, at the time, Newton was trying make some anology with the musical scale and the octave (with its seven intervals) and hence was keen to identify seven colours in the rainbow or visible spectrum. Many modern commentators claim that only six distinct colours can be observed in the rainbow.

Interestingly, the facebook group referred to above would like to eject indigo from the spectrum on the basis that it is not a primary or secondary colour but rather a tertiary colour. The group shows the following colour wheel:

colour wheel

In this so-called painters’ wheel the primary colours are red, yellow and blue and the secondary colours are orange, green and violet. It is argued that since six of the colours in the rainbow are primary or secondary colours in the colour wheel and indigo is not, then indigo has no right to be there. This is wrong on so many levels it is hard to know where to start.

The first thing I would have to say is that this argument seems to ignore the difference between additive and subtractive mixing. Additive mixing – http://colourware.wordpress.com/2009/07/13/additive-colour-mixing/ – describes how light is mixed and the additive primaries are red, green and blue. The additive secondaries are cyan, magenta and yellow. Orange is not in sight – and yet surely if we are to make an argument for inclusion in the spectrum based on primaries (and/or secondaries) then it is the additive system that we should be using since the spectrum is emitted light.  

The optimal subtractive system primaries are cyan, magenta and yellow (with the secondaries being red, green and blue) though the artists’ colour wheel (which is like the painters’ wheel above) has red, blue and yellow as the primaries. 

In my opinion there is nothing special about the colours that we see in the spectrum. Indeed, orange is clearly a mixture of red and yellow and does not seem to me to be a particularly pure colour. I just do not think that arguments to exclude indigo from the spectrum based upon colour wheels or primary colours is valid. That said, I have already mentioned that many people believe that indigo cannot be seen in the spectrum as a separate colour; but this is a phenomenological observation not dogma. I am one of those who believe that indigo and violet cannot be distinguished in the spectrum and therefore I agree with the aims of the facebook group even if I do not agree with their arguments.

The really interesting question is why we see six (or even seven) distinct colour bands in the spectrum when the wavelengths of the spectrum vary smoothly and continuously? I have postulated some possible reasons for this in an earlier post – http://colourware.wordpress.com/2009/07/20/colour-names-affect-consumer-buying/ – but it is far from a complete and convincing explanation. It may explain why we see distinct colours in the rainbow, but why six and why those six in particular. Comments on this would be very very welcome.

CIE system of colorimetry

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For about 100 years there has been an international system for colour specification – it’s called the CIE system. The acronym comes from Commission Internationale de L’Eclairage.

This system is based on the notion of additive colour mixing – http://colourware.wordpress.com/2009/07/13/additive-colour-mixing/

Since it is possible to mix together three primary lights and make a wide gamut of colours (though not, of course, all colours) the principle is that the amounts of these primaries that an observer would use to mix togther to match a colour is a useful specification of that colour. We refer to these amounts as tristimulus values. One could imagine a visual colorimeter whereby an observer would try to match a colour that is to be specified by adjusting the intensities of three primary lights that are mixed together – once a match is obtained then the tristimulus values would define or specify the colour. All that would be necessary would be to able to decide on a set of primaries and manufacture the visual colorimeters so that they are very consistent from one device to the next. It would be a little clumsy though to have to use one of these visual colorimeters. But in principle it could work.

Fortunately the CIE does not require the use of such visual colorimeters since in 1931 the CIE measured the trismumulus values that observers made when matching various colours. These were averaged to create the so-called CIE standard observer.  And here’s the really clever bit. Having defined the CIE standard observer it is possible to calculate the tristimulus values (the amounts of the three primaries that an observer would use to match a colour) without any further observations. All that is required is that we know the amount of light at each wavelength reflected by a sample or (in some cases) emitted from a device such as computer display and then – by using our knowledge of the CIE standard observer – it is possible to calculate the tristimulus values.

So what were the primaries. If you have read my previous post, What is a colour primary – http://colourware.wordpress.com/2009/07/08/what-is-a-colour-primary/ – you’ll know that the choice of colour primaries is somewhat arbitrary. Well, in fact the original determination of the standard observer what carried out in England using red, green and blue primaries. But the data obtained were later modified to refer to a different set of primaries known as X, Y and Z. It was necessary to make this adjustment because using any set of real primaries it was impossible to match any colour with mixtures of the primaries; using RGB meant many colours could be matched, but not all. So a set of so-called imaginary primaries was conceived which could – in theory – be used to match all colours. So the tristimulus values of the CIE system are known as X, Y and Z. 

In fact, it didn’t really matter which set of primaries was used; the CIE system was concerned with colour matching. If two samples have the same tristimulus values then they would be a visual colour match no matter which set of primaries was used. So the choice of primaries really was not critical.

Today many instruments are commercially available – colorimeters, reflectance spectrophotometers, radiometers) – that, with the use of software, allow the CIE XYZ values to be measured; these instruments are extremely valuable in many industrial and commercial applications. The CIE system is still very much alive today, though many users often prefer to use one of the more advanced colour spaces – such as the CIELAB colour space – which was defined by the CIE in 1976 and whose values are very easily calculated from the CIE XYZ values.  For further information about the CIE please visit their web site – http://www.colour.org/

additive colour mixing

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There are – broadly speaking – two types of colour mixing: additive colour mixing and subtractive colour mixing. Subtractive colour mixing relates to how inks, paints, dyes etc add together to form different colours; additive colour mixing refers to how light-emissive colour devices create colours. So we’re talking about how computer monitors work or how phone displays work.

The essential principle behind additive colour mixing is that we can mix together three colours – called colour primaries – and create a surprising range of colours. See my earlier post – http://colourware.wordpress.com/2009/07/08/what-is-a-colour-primary/ – for further details about colour primaries. The additive primaries are red, green and blue. Is there anything special about these three colours that justifies their use as the primaries? No, apart from the fact that if you use red, green and blue as the additive primaries you get a large gamut (range of colours that can be produced).  There is no reason why you couldn’t use orange, purple and turqiose as the additive primaries – it’s just the range of colours that could be created would be unsatisfactorily small. And nobody would like that!

So, we have red, green and blue as the additive primaries. The figure below illustrates how additive colour mixing works. Imagine that we have three projection lamps at the back of a hall – one has a red filter and so produces a beam of red light, and the other two use filters to produce green and blue beams. We project these onto a white screen and get three circles of light (one, red, one green and one blue). We then move the angles of the projectors so that the circles of light overlap. We get something that looks rather like this:

additivemixing_b

Where the red and green light overlap we get yellow. We get magenta and cyan for the other two binary mixtures. So,

red + green = yellow

red + blue = magenta

green + blue = cyan

And if we mix all three primaries we can achieve white (or other neutral colours). The primaries could be single wavelengths of light – so we could use a primary at, say, 700 nm (for the red) and one at 450 nm (blue) and one at 530 nm (green). In practice, most devices (CRTs, LCDs etc) don’t use single-wavelength primaries since it would be hard to create bright screens (gamuts are 3-D not just 2-D) but in principle could do so. It’s also important to note that different devices and different manufacturers use slightly different primaries.

But let’s imagine for a second that the three primaries used in the pictuire above are at 450 nm, 530 nm and 700 nm. Green light (530 nm) and red light (700 nm) additively mix together and generate yellow. When this happens what is being mixed and where does this mixing take place? Take a few moments to consider this before reading on.

Notice I said that they additively mix to generate yellow – I specifically avoided saying that they mix to generate yellow light. If we look at the part of the screen that is yellow we would see that we have some light at 700 nm and some at 530 nm. The wavelengths are not mixed; they don’t mix together to generate some third wavelength of light such as 575 nm (I choose this wavelength since monochromatic yellow light is about 575 nm). So no physical mixing takes place other than – I suppose one could argue – that the red and green lights are mixed in the sense that they are spatially coincident. But that’s not really mixing, for me, and certainly doesn’t even begin to explain why we have the sensation of yellow when we look at these wavelengths together.

So when we say that the red and green lights are mixed together to create yellow we should be aware that no phsyical mixing takes place. Indeed, one could argue that mixing is really the wrong word to use. Though as I write this I am struggling to think of a better one – suggestions on a postcard please.

When we look at the mixture of red and green light we see yellow – but the eye is still receiving the indivual wavelengths of red and green light. However, the visual response to this is that yellow is perceived. Indeed, a carefully composed mixture of red and green light could produce a yellow that is visually indistinguishable from yellow monochromatic light; but physically the mixture would still consist of light at 530 nm and light at 700 nm. If mixing occurs at all in any real sense it is in the perceptual mechanisms of the visual system. Indeed, at the heart of this matter is the way in which our visual pigments respond to light …. more about that another time.

What is a colour primary?

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I try to keep my blogs short. But this is going to be difficult!

Let’s start with what a primary is not. The primaries are often quoted as being red, yellow and blue. And on the BBC website – of all places – it says “Primary colours are three key colours – Red, Yellow and Blue. They cannot be made from any other colour”. For the BBC site visit http://www.bbc.co.uk/homes/design/colour_wheel.shtml.

The statement that primary colours cannot be made from any other colour is simply not true. Further, it gives the mistaken impression that there is something special about these primaries (red, yellow and blue) that makes them stand apart from the other colours. Another false statement that one often sees even in quite scholarly work is that any colour can be made by the mixture of three primary colours.

What is true is that if we select three appropriate colours and mix them together we can make a suprising range of colours. These colours – let’s call them primaries – can combine to make a huge range of different colours but unfortunately there are no three primaries that can be selected such that in mixture they can create any other colour. Depending upon the choice of primaries the range (or gamut) of colours that can be created in mixture is larger or smaller. What makes a good set of primaries? Well, I think it is reasonable to say that a good set of primaries is one where the gamut of colours that can be created is large; indeed, I would argue that the optimal primaries are those that create the largest possible gamut.

There are certain sets of primaries that can easily be predicted to give a small gamut. For example, if any of the primaries are dull and desaturated then the gamut will not be very big. Also, if the three primaries are similar to each other then the primaries are not likely to be a good set. And finally, if it is possible to combine two of the primaries together to make the other one then the gamut will be tiny. This is where – I believe – the misleading statement on the BBC website comes from; for a good set of primaries it is important that the primaries are independent (that two cannot be mixed to match the other one) but this is a long way from the BBC statement. Once we havs selected three suitable priamries then it’s true that they cannot then be made by any other colours that the primary system can make – but to argue that this is why they are primaries is clearly a circular argument.

There is nothing special about red, yellow and blue. In fact, they are not even the optimal primaries! To say what the optimal primaries are we need to specify the type of mixing: additive or subtractive. Additive mixing describes the behaviour of light-emissive systems such as computer displays, subtractive mixing describes how paints and inks mix. For subtractive mixing the primaries are often quoted as red, yellow and blue, as in the BBC article. However, a larger subtractive gamut is obtained if we use cyan (instead of blue) and magenta (instead of red) – see figure below.

primary 

One of my current research interests is to understand why red, blue and yellow became known as the artists’ primaries when a larger gamut is obtained if a bluish red is used (something closer to a magenta) and if a greenish blue is used (something closer to cyan). One only has to look at the primary colours used in inkjet printers for example (where the manufacturers have a vested interest in being able to create a large gamut) to realise that cyan, magenta and yellow are the optimal subtractive primaries.

For additive colour mixing the optimal primaries are red, green and blue. The additive and subtractive primaries have an interesting relationship – but that’s for another blog, another day.

Is black a colour?

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This is one of the most frequently asked questions I come across. 

For me there is no doubt that black is a colour whether argued from an objective or subjective position (see http://colourware.wordpress.com/2009/07/03/does-colour-exist/).

If we take an objective position and argue that colour results from pigments or dyes in the world then we should note that black most commonly occurs with the pigment carbon black, an efficient absorber of light at most visible wavelengths. I see no categorical distinction between carbon black and other pigments (e.g.  prussian blue, malachite green, titanium doixide). Those objectivists who associate colour with light and hence argue that black occurs in the absence of light and is therefore not a colour, can be countered with my observation that in over 25 years I have never seen an object that absorbs all of the light that falls upon it;  though it is my understanding that no light ‘escapes’ from black holes.  But I have seen lots of black things. In fact, most black objects reflect at least 5% of the light that falls upon them.  If you look at an old CRT screen turned off the screen would look grey (not black). However, when turned on, black objects in the CRT image would look far blacker than the screen when it was turned off though they obviously cannot be emitting less light. The enhanced black is caused, of course, by contrast. Modern LCD screens are less prone to this effect and I believe this is the result of more sophisticated coatings that mean that the screen reflects less of the ambient illumination. For a nice example of contrast, see the image below; the four small grey rectangles are all physically the same but appear to be lighter as we move from right to left.

contrast1

This leads us to the subjective view of colour. Colour is a perception. In this sense I see no reason to distinguish between black, white, grey, red and blue. That is not to say that there are no differences between these colours. White, black and grey are, as we know, achromatic whereas red and blue are chromatic colours of a specific hue. To argue that black is not a colour in this sense, however, would lead one to question whether white and grey are colours. Of course, some people take this position. There is a tradition in visual neuroscience to separate so-called colour processing from lightness processing in the visual system because they are believed to result from distinct neurophysiological processes. However, a modern understanding of colour perception is that colour is a three-dimensional percept; the dimensions being hue, colourfulness and brightness.  I would therefore argue that black is not the absence of light since black often (in fact, usually) occurs when light is being reflected or emitted; however, black is an achromatic colour like white and grey.

Does colour exist?

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I would like to discuss the issue of whether colour exists or not, from a philosophical perspective. Speaking more strictly I am going to be writing about the nature or ontology of colour, since to argue that it doesn’t exist at all would be somewhat damning on my career as a colour scientist to date.

Before you continue I suggest you make yourself a strong black coffee, dim the lights, and relax to avoid the thumping headache that could result from reading further without taking these precautions.

In a previous blog (http://colourware.wordpress.com/2009/06/29/colour-101/) I wrote about the putative relationship of wavelengths of light with colour. One view of colour ontology is objectivism; that is, that objects are coloured and that the colours of objects can be identified with the composition (wavelengths) of light reflected or with the reflectance factors of objects. Certain wavelengths can be associated with certain colours and objects have certain colours because they reflect certain wavelengths of light and absorb others. Simply put a red object is red because it absorbs the short wavelengths of light and reflects (or transmits) the longer wavelengths. However, it is easy to show that the colour of an object (for example, a patch in a scene) is not invariant; rather, it changes with the surrounding or background colours (as shown below).

contrast

In this example, the two central squares are physically identical in their spectral properties but appear to be different colours. Metamerism would also seem to be troublesome for objectivism. Metamerism commonly occurs, for example, when two objects reflect different wavelengths compositions but are indistinguishable in colour when viewed under a particular light source; crucially, when seen under some other light source the two objects no longer match each other in colour. We also know that the same object will look different in colour to a so-called colour blind observer (~10% of the male population are colour blind) compared with a so-called normal observer. Objectivists could counter this by saying that objects are coloured and one can equate colour with physical properties – it is just that we need to define standard conditions. But there is great difficulty in defining what those standard conditions are.

Thus, although some philosophers still argue for colour objectivism, many reject it – including, for example, Evan Thompson who wrote a fantastically entertaining and informative book on this very subject (http://www.amazon.co.uk/gp/search?index=books&linkCode=qs&keywords=0415117968). It would seem that when Newton famously wrote that ‘the rays are not coloured’, he was also rejecting objectivism though there is some lack of clarity from Newton’s writings on this matter.

The natural opposite of objectivism is subjectivism; this takes the view that things are coloured only in so far as they have the disposition to cause sensations of colour in a perceiver.

An extreme form of subjectivism is called extremism; according to this view nothing is strictly speaking coloured at all, not even dispositionally. Colours are entirely in the head; they are nothing but sensations of a certain type. This is the view that I adhere to. Colour is a sensation that results from a biological process that occurs in our brains and, presumably, in the brains of many other species. I was once challenged by a famous American lawyer on this point; it’s a long story why this happened, but suffice to say he asked me whether I believed that if a tree fell in a forest and there was nobody there would it make a sound? Although at the time I managed to side-step this difficult question I can state here that I do not think it would make a sound. I believe that when an object ‘makes a sound’ it causes wave-like vibrations in the air and that our auditory systems detect these vibrations, convert them into neural signals, and ultimately result in a neural state that results in the listener experiencing a sound. Without a listener there can be no sound. Similarly, objects reflect wavelengths of light, these wavelengths are detected by our visual systems … and we experience colour. To me, a planet without life would have no colour, no sound, no taste etc and arguing otherwise is like arguing that that planet would have pain or fear.

However, it is not straight forward that if we reject objectivism we should embrace subjectivism. There are arguments that can be used to reject the extreme form of subjectivism that I believe in. Although at first it may seem obvious that colours are either properties of objects or ‘in our heads’ Thompson suggests that colours could be relational properties, not intrinsically linked to any item. According to this view there would be no perceiver-independent account of colour but neither would colours be reduced to mental or neural states. Rather, colour would be a relational property, resulting from the relationship between objects and observers.

Ultimately I believe it is possible to make a case for any of these views about the ontology of colour. The truth may well be somewhere between objectivism and subjective extremism. So why should I be so passionate about arguing for subjective extremism? The answer is that over 20 years of teaching students about colour has led me to the view that the notion that colour is a fixed and invariant property of objects is a barrier to their learning. This notion that they have originates, and is constantly reinforced, by our use of language when we say, for example, that “this book is red” or “that pencil is yellow”. Whenever they come across a situation when an object seems to change colour (as I have shown, this happens when we change the colour of surround and in many different situations) they dismiss it as an illusion. This prevents them from easily understanding some important concepts in colour education.  

Thus I would say that when an objects changes colour because of the background or the light source, it’s not an illusion. Rather, it’s an illusion to think that objects have a fixed colour.