Monthly Archives: July 2009

Rose-tinted lenses

In a previous post – – I expressed sceptisism about whether coloured contact lenses or spectacles can improve colour discrimination in colour-blind observers. Today I came across a related story about the use of coloured lenses to improve reading in some people suffering from migraines and/or learning difficulties. It seems to be news in Australia – – but I have been aware of research that coloured transparencies can help with reading for many years now. Apparently certain people respond better with certain colours – but I wonder why?

additive colour mixing

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 – – 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:


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.

Society of Dyers and Colourists

I joined the Society of Dyers and Colourists in about 1983 when I was undertaking my doctorate at Leeds University. I mainly joined for the fun to be honest, but as a serious young student I think I thought it may have some positive effect on my career. Little did I know that I would be still a member some 25 years later and that during that time the SDC would have been a very important part of my professional life. Through the SDC I have gained technical knowledge (through my membership of its technical committees such as the Colour Measurement Committee) but also met many kind and knowledgeable people in the world of colour. And I’d like to think I have put something back too through my role on the editorial board of the journal Coloration Technology and, more recently through my role as founding academic editor of the SDC online journal Colour: Design and Creativity.

I would encourage anyone with an interest in colour technology or colour design to have a look at the SDC. They are based in Bradford but have an international presence and hold meetings all over the world.

Visit their web site 

The SDC even has its own blog –

What is a colour primary?

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

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.


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?

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

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.


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.

Energy-aware colour palette

Flat-panel displays based on organic LEDs are likely to become commonplace soon, replacing LCD panels, because they are more energy efficient. However, another potential advantage is that OLEDs use more or less energy depending which colours (hues) are being displayed. This is in contrast with LEDs which use the same amount of energy no matter which hue is being displayed. Research by Johnson Chuang at Simon Fraser University in Canada suggests that the selection of an energy-aware colour palette could save battery life on mobile devices. For further details see

colour blindness news

When we say someone is colour blind its a misnomer, since most people who are colour blind can see colour; it’s just they have poorer colour discrimination compared with so-called normal observers. Colour-blind observers will confuse two colours, for example, that would normally be easilly discriminated between. Very often, but not always, it is reds and greens that are confused. Colour blindess affects about 1 in 10 of the male population but is very rarely found in females.

Colour blindness that is inherited genetically and is present from birth is normally considered to be incurable. However, there are contact lenses on the market that claim to improve colour vision for colour-blind people. A recent news story concerns a man in the UK who is testing out one of these products and sharing his experiences in the Daily Mail. For the full story see

The gentleman in question has always wanted to be qualified to fly an aeroplane. However, any claims that contact lenses can improve colour vision should be treated with caution; they may upset the delicate balance of a colour-vison test and enable someone to pass a colour-vision test but this doesn’t mean that they bestow normal colour vision on the wearer.

At the same time, a new colour vision test developed by John Barbur at City University (London) was commissioned by the UK’s Civil Aviation Authority (CAA) that could allow colour-blind people with only a mild deficiency to be distinguished from those with more serious colour-vision problems. This could open up the door to some colour-blind people being allowed to have occupations that they previously would have been ineligibe for. Further details can be found on the BBC web site:

Does colour exist?

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 ( 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).


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 ( 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.

moths change colour again!

We all know the story of the peppered moth that changed its colour from white to black in certain areas of the UK in response to increased pollution. The change made it less obvious to predators against backgrounds of grime and soot.


Well, apparently, 200 years later in post-industrial Britain it’s changing back to it original colour. For the full story go to

People are being urged to look out for moths, note their colour, and log them at a special web site –

ps. Apparently moths are not the only thing changing colour – some people say that the white backs of the Apple iPhone 3GS are changing colour with frequent use. Of course, I couldn’t possibly comment