Tag Archives: knowledge
Where is colour mixing?
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
This is called additive colour mixing as I am sure you know. 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). So 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. When I sat down with a couple of students last week and asked then what they though they said that the red and green light mixed together to create yellow light and when I pressed them, they went further to say that the yellow light was at about 575 nm.
If we measure 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. 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. It also makes me think that additive colour mixing, if it can be said to occur anywhere in particular, occurs in the eye. And I do mean eye, not brain.
is there such a thing as visible light?
I would argue that there is no such thing as visible light – or at least that the term visible light is a meaningless one.
Light is part of the electromagnetic spectrum which is describes electromagnetic radiation by its wavelength. An electromagnetic wave has both electric and magnetic field components. What is really very interesting is that depending upon the wavelength of the field the electromagnetic radiation has very different properties and we give it a different name.
When the wavelength is very long, the radiation is radio waves or micro waves. When the wavelength is very short, the radiation is x-rays or gamma rays. There is a narrow range of wavelengths (from about 360 nm to about 780 nm – a nm is 0.000000001 of a metre) to which our eyes are sensitive. Because we can literally see this radiation we call it light. I still find it amazing that light, x-rays, radio waves, and microwaves are all essentially the same thing (electromagnetic radiation) with just a change in the wavelength!! However, my point for today is that light is radiation that is visible – to talk about visible light would be bizarre since by its very definition light is visible. Technically, visible light is a pleonasm; pleonasm is a word derived from the Greek word “pleon” meaning excessive. Other examples of pleonasms – easily confused with oxymora – include the phrases end result and invited guests.
Colour Semiotics – a personal view
Colour is an important component of many successful designs. It is interesting, therefore, to consider why certain colours are chosen in designs and under which circumstances the colour choices enhance the likelihood that the design will be successful. In this paper, four aspects of colour design (colour harmony, colour preference, colour forecasting and colour semiotics) will be briefly considered and one of these, colour semiotics, will be explored in some detail. Finally, the role of all four of these aspects of colour in the design process will be discussed.
Colour Harmony
Colour harmony is concerned with the relationship between colours. One definition of colour harmony is that it refers to when two or more colours are seen in neighbouring areas that produce a pleasing effect (Judd & Wyszecki, 1975). Many theories of colour harmony are ideological in nature and Itten wrote, for example, that ‘One essential foundation of any aesthetic color theory is the color circle, because that will determine the classification of colors’. In the last 150 years, Rood (1831-1901), Ostwald (1853-1932), Munsell (1858-1918), Itten (1888-1967) and others proposed various theories that were based on certain geometric relationships in a colour circle (or more generally in a colour space) being harmonious (Westland et al., 2007). For example, colour combinations whose representations in a colour space form the vertices of a triangle are considered to be harmonious according to some theories. Most of these theories were based on personal introspection and a belief that classical geometric shapes should frame the colour relationships that are harmonious but there is no a priori reason why this should be. Moreover, there have been few studies to robustly test whether theories of colour harmony can be justified empirically. However, when referring to colour harmony it is not always clear that authors are even referring to the same thing. Colour harmony has been used to refer to colours being pleasing, harmonious, and successful. In addition, it is generally accepted that ideas about colour harmony shift over time (Nemcsics, 1993) with fashion and taste and this has led some to claim that “It is quite evident that there are no universal laws of (colour) harmony” (Kuehni, 2005). Nor is it even clear that laws are even required since the majority of designers and artists naturally are able to select colour combinations that are harmonious (by whichever definition) without assistance. It is therefore, perhaps, useful to place colour harmony in the field of aesthetics.
Colour Preference
Colour preference is also best placed in the field of aesthetics but is generally used to refer to a single colour – though the distinction between colour harmony and colour preference is being explored by the work of Ou and colleagues (e.g. Ou et al., 2004b). An early study was carried out by Guildford and Smith (1959) who asked 40 observers to assess the pleasantness of each of 316 Munsell samples according to an 11-point scale (where 0 and 10 corresponded to the least and most pleasant colours imaginable respectively). This study, like most others since, revealed a preference for blue and green colours and a dislike of yellow (on average, of course; individual results usually vary greatly). More recently, 208 participants undertook a simple forced-choice ‘color-picking’ task and the data revealed a robust cross-cultural sex difference (Hurlbert and Ling, 2007) with females’ hue preferences shifted to longer wavelengths when compared with those of males. Hurlbert and Ling suggested the sex differences may be linked to the evolution of sex-specific behavioural uses of trichromacy. Schloss and Palmer also recently studied colour preferences and found that despite, on average, participants preferring yellow hues to blue hues there was considerable variability between individual colour preferences. They proposed an ecological valence theory that suggests that people prefer colours that are associated with objects and situations that are affectively positive for them (Schloss and Palmer, 2010). However, in all of these studies, when observers are asked which colours they prefer it is not clear that they always respond with the same purpose in mind (that is, in what sense or context are the observers judging preference?).
Colour Forecasting
Colour forecasting is a particular phenomenon that relates mainly, but not exclusively, to the textiles fashion and interior design fields (Diane and Cassidy, 2005). It involves the prediction of future colour trends via an appraisal of past colour trends and an assessment of lifestyles associated with these trends. It is not at all clear that colour forecasting is a forecasting or predictive process at all and there is no empirical evidence that colour consumption is influenced by socioeconomic lifestyle factors at all (Stansfield and Whitfield, 2005). Despite this, colour forecasting is an important component in many colour-production industries. It could, however, be argued that colour forecasting should be placed in the field of marketing since the process could be argued to be more about telling consumers which colours they wish to purchase rather than predicting which colours consumers would like to purchase.
Colour Semiotics
Colour semiotics is concerned with the meanings that colours are able to communicate. Colours can evoke strong emotional responses in viewers and can also communicate meanings and or concepts through association. For example, in many western societies black is associated with death and the mourning process. Consequently, colour may play a role in imparting information, creating lasting identity and suggesting imagery and symbolic value (Hynes, 2008). There seem to be at least three different origins for colour semiotics. Firstly there is the emotional or visceral impact of colours. Colours can have a strong emotional impact and can even affect our physiological state. For example, red colours have been cited to raise the blood pressure and colours have been reported to affect muscular strength (Hamid and Newport, 1989; O’Connell, Harper and McAndrew, 1985). We fear the dark. Perhaps these effects are innate and have been present since the earliest days (the effect of red has sometimes been attributed to the colour of blood and our fear of black may relate to a primitive fear of the dark and unknown.) Secondly there are socio-economic origins. In western society purple became associated with wealth and royalty because purple dyestuff was more expensive than gold. Only extremely rich people could afford to wear purple and some organizations (e.g. the Christian church) chose to use purple to make a statement about their wealth and power. Thirdly, some colours meanings are cultural in origin. The association of red with luck in China and the link between pink for girls and blue for boys in western society may originate in and be reinforced by cultural behaviour and shared understanding. For example, in the United Kingdom pink was associated with young boys until about 1920 after which blue came to signify the male professions, most notably the navy (Koller, 2008). The importance of colour semiotics has been noted in corporate visual identities (Hynes, 2008), human computer interaction (Bourges-Waldegg and Scrivener, 1998), political communication (Archer and Stent, 2002), and as a marker for gender and sexuality (Koller, 2008). Koller undertook a study of the colour pink and found, from a survey of 169 participants, that 76 per cent of participants made the association of pink with femininity. Pink was also associated with romance (56%), sweetness (52%), softness (51%), love (50%) and several other concepts (Koller, 2008). Men were less likely to make synesthetic associations for pink than were females who also seemed to have a more differentiated schema for pink. In addition to the link between pink and femininity, Koller (2008) also found emergent associations of pink with fun, independence and confidence. However, although black is often associated with death it can have other meanings; for example it can be associated with power or evil, and the actual meaning in any particular situation depends upon the context in which the colour is used; it can also depend upon other aspects of visual appearance such as gloss and texture (Lucassen, Gevers and Gijsenij, 2010). Furthermore, the meanings for a colour can also depend upon culture and can vary over time. For example, in some countries black is not the colour that is most associated with death (white is used instead). The appropriate use of colour semiotics can impact greatly on the success of a design (particularly one that has a branding or marketing dimension). However, it is clear that colour meanings and associations can vary with a great many factors. On the one hand the connection of meaning and colour seems obvious, natural nearly; on the other hand it seems idiosyncratic, unpredictable and anarchic (Kress and Van Leeuwen, 2002). Indeed, social groups that share common purposes around colour are often relatively small and specialized compared to groups who share speech or visual communication (Kress and Van Leeuwen, 2006). Grieve goes further to suggest that colour per se does not elicit response, but the particular meaning or significance of the colour seems context-bound and varies from one person or situation to another (Grieve, 1991).
Despite the previously discussed context–‐dependence of colour semiotics most robust studies that have explored colour semiotics have done so for colour patches viewed in an abstract sense, devoid of context. The colour science community tend to use the term colour emotion instead of colour semiotics; for example, Gao et al. (2007) wrote that “The semantic words describing words such as “warm-cool”, “light‐dark”, “soft‐hard”, etc.”. The colour science community also tend to study bi-polar pairs of semantic words such as “soft-hard”. In these circumstances it has been found that there is an effect of culture but that it is limited (Lucassen et al., 2010). Indeed, even the medium (e.g. digital display or hardcopy paper) has been shown to have little effect on the emotions or meanings that observers attribute to different colours (Suk and Irtel, 2010). This would seem to contradict greatly with the earlier view (Grieve, 1991) that colour per se (without context) does not elicit response. Nevertheless, most formal studies in the last decade have explored whether there are cultural, gender or age effects in terms of the meanings associated with colours by observers when viewing colours without context (typically square patches of colour viewed on a computer screen). For example, one study (Gao et al., 2007) studied observers from seven countries (Hong Kong, Japan, Thailand, Taiwan, Italy, Spain and Sweden) who were asked to rate 214 colour samples each in terms of 12 bi-polar word pairs (e.g. soft-hard). The differences between the nationality groups were small despite the different cultural backgrounds. In another study (Ou et al., 2004a) 14 British and 17 Chinese observers assessed 20 colours in terms of 10 bi‐polar word pairs. The differences between the responses from the two groups were small with the exception of like‐dislike and tense-relaxed. Chinese observers tended to prefer colours that were clean, fresh or modern whereas this tendency did not occur for British observers. British observers tended to associate tense with active colours, whereas Chinese observers associated tense with the colours that were hard, heavy, masculine, or dirty. In a second study (Ou et al., 2004b) 8 British and 11 Chinese observers assessed 190 colour pairs in terms of 11 bi-polar word pairs. No significant differences were found between the UK and Chinese responses but some gender differences were found; there was poor correlation between male and female responses in terms of the masculine-feminine word pair and female observers tended to like colours that were light, relaxed, feminine or soft (whereas this association did not occur for male observers). It seems clear that colour per se does have meaning but the question of whether these meanings are consistent across culture, age and gender is not entirely clear. As Gage (1999) wrote, “To what extent different colours, such as red or black, have cross-cultural significance, is an altogether more difficult question.” Perhaps one reason why these formal studies have not been able to provide definitive answers to the question of whether colour meaning and emotion depends upon culture (and even gender) is because they have traditionally been carried out with quite small numbers of participants. The two studies by Ou et al. (2004a; 2004b) involved 31 and 19 participants respectively. These studies typically involved small numbers of observers in part because the experiments are carried out in laboratories using carefully controlled and calibrated equipment so that the exact specifications of the colours displayed can be known. One way to involve much greater numbers of participants is to use a web-based experiment and such a study is currently being undertaken by the author (Westland and Mohammadzadeh, 2012). Web–‐based experiments have several advantages including access to large numbers of observers and minimal interruption to observers and experimenter. Of course, the disadvantages are also numerous including potential sources of colour variation including, display technology, ambient illumination level, observer bias an, deficiencies and anomalies and operating software. However, currently responses have been collected for more than 2000 observers from over 50 countries worldwide and this work, when complete, has the potential to allow definitive conclusions to be drawn on the question of whether colour semiotics are invariant to cultural background and gender. The issue of how to address colour semiotics in a design context remains an open question and can currently only be addressed by ad hoc studies that contribute little to the theoretical debate.
References
Judd DB and Wyszecki G (1975), Color in business, science and industry, 3rd edition, John Wiley and Sons.
Westland S, Laycock K, Cheung V, Henry P and Mahyar F (2007), Colour Harmony, Colour: Design and Creativity, 1 (1), 1-15.
Nemcsics A (1993), Colour dynamics: Environmental colour design, Ellis Horwood.
Kuehni RG (2005), Color – An introduction to practice and principles, John Wiley and Sons.
Ou L-C, Luo MR, Woodcock A and Wright A (2004a), A study of colour emotion and colour preference. Part I: Colour emotions for single colours, Color research and application, 29 (3), 232-240.
Guildford JP and Smith PC (1959), A system of color preferences, American Journal of Psychology, 72 (4), 487‐502.
Hurlbert AC and Ling Y (2007), Biological components of sex differences in color preference, Current Biology, 17 (16), R623‐R625.
Schloss KB and Palmer SE (2010), An ecological valence theory of human color preference, Proceedings of the National Academy of Sciences, 107 (19), 8877-8882.
Diane T and Cassidy T (2005), Colour Forecasting, Wiley-Blackwell.
Stansfield J and Whitefield TWA (2005), Can future colour trends be predicted on the basis of past colour trends?: An empirical investigation, Color research and application, 30 (3), 235‐242.
Hynes N (2009), Colour and meaning in corporate logos: An empirical study, Journal of Brand Management, 16 (8), 545‐555.
Hamid PN and Newport AG (1989), Effect of colour on physical strength and mood in children, Perceptual and Motor Skills, 69, 179‐185.
O’Connell BJ, Harper RS and McAndrew FT (1985), Grip strength as a function of exposure to red or green visual stimulation, Perceptual and Motor Skills, 61, 1157-1158.
Archer A and Stent S (2002), Red socks and purple rain: the political uses of colour in late apartheid South Africa, Visual Communication, 10 (2), 115-128.
Koller V (2008), ‘Not just a colour’: pink as a gender and sexuality marker in visual communication, Visual Communication, 7 (4), 395‐423.
Bourges‐Waldegg P and Scrivener SAR (1998), Meaning, the central issue in cross–‐cultural HCI design, Interacting with computers, 9 (3), 287‐309.
Lucassen MP, Gevers T and Gijsenij A (2010), Texture affects color emotion, Color research and application, 36 (6), 426‐436.
Kress G and Van Leeuwen T (2002), Colour as a semiotic mode: Notes for a grammar of colour, Visual Communication, 1 (3), 343‐368.
Kress G and Van Leeuwen T (2006), Reading images: The grammar of visual design, Routledge. Grieve KW (1991), Traditional beliefs and colour perception, Perceptual and Motor Skills, 72, 1319-1323.
Gao X-P, Xin JH, Sato T, Hansuebsai A, Scalzo M, Kajiwara K, Guan S–‐S, Valldeperas J, Lis MJ and Billger M (2007), Analysis of cross–‐cultural color emotion, Color research and application, 32 (3), 223-229.
Suk H‐J and Irtel H (2010), Emotional response to color across media, Color research and application, 35 (1), 64-77.
Ou L‐C, Luo MR, Woodcock A and Wright A (2004b), A study of colour emotion and colour preference. Part II: Colour emotions for two-colour combinations, Color research and application, 29 (4), 292-298.
Gage J (1999), What meaning had colour in early societies?, Cambridge Archaeological Journal, 9 (1), 109‐126.
Westland S and Mohammadzadeh M (2012), http://www.keysurvey.co.uk/votingmodule/
good to be colour blind
Colour blindness is mainly a male affliction. Something like 8% of all men in the world are colour blind though, as I have mentioned before, this doesn’t mean that they cannot see colour but, rather, means that their colour discrimination is not as good as that of so-called normal observers (the rest of us, in common vernacular). See my earlier post. So we normally think of colour blindness as being something undesirable, something that ideally we would like to be able to cure.
Interesting then that new research at Anglia Ruskin University has suggested that colour blindness may even be an advantage. The study was led by Dr Andrew Smith and showed that colour-blind monkeys (tamarins, to be exact) were better than their ‘normal’ counter-parts at catching camouflaged insects (such as crickets). I guess what this means is that the camouflage is designed (I guess I should say, has evolved) to be effective when viewed by normal tamarins. So the colour-blind tamarins may be better off in some sense.
Dr Smith is also quoted as saying that there is some evidence that, in humans, dichromats (who have two classes of cone rather than three) may see better in dim light than trichromats. For further information see http://www.businessweekly.co.uk/academia-a-research/13403-colour-blind-monkeys-have-advantage-in-catching-camouflaged-prey.
What is a colour space?
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:
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.
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.
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).
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.
WebExhibits colour site
I just came across a really interesting site – see http://www.webexhibits.org/
It includes some great interactive colour-related ideas. It looks like a fantastic teaching resource.
Indigo – a colour of the rainbow?
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:
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.
Colour 101
I am really looking forward to some interesting topics such as
Is black a colour?
and
Does colour exist?
But, before I get into these tough topics I would like to present some basic and rudimentary notions about colour and what it is. Look in any textbook on colour and you’re almost certain to find a picture of the electromagnetic spectrum looking something like this:
It was Newton, of course, who famously studied the relationship between wavelength and colour. Light is a form of energy called electromagnetic radiation. Light can be characterised by its wavelength and our visual systems are sensitive to wavelengths in the approximate range 400-700nm (we’ll deal with the exact wavelength range later). So we call radiation in this range the visible spectrum or, more simply, light. In my diagram above the short wavelengths are on the right and the longer wavelengths on the left. So we might simplistically think that, for example, light at 400nm is blue or violet and that light at 700nm is red. It’s nowhere near as simple as this but it would do no harm to think that way for the present.
The spectrum above raises two interesting questions straight away however. The first is, why – since the wavelength of light varies continuously from about 400nm to about 700nm – do we see these specific and discrete colours? When I was at school I learned the mnenomic Richard Of York Gave Battle in Vain to remember the order of the colours in the spectrum. But why don’t we see a continuous range of colours – or, to be technically more precise – hues? The answer is something called categorical perception. However, just as interesting is my second question. Why do the two ends of the spectrum look rather similar. OK, red and violet are not the same. But certainly, red is closer to violet perceptually than it is, to say, green. And yet in wavelength terms red is closer to green! I’ll be returning to this issue of circularity of hue in a later post. However, if you would like to explore either of these phenomena yourself then I would encourage you to spend time looking at a rainbow. When sunlight strikes droplets of water in the air (this often happens on a sunny day after a rainstorm) the wavelengths separate (a process called refraction) and we see the visible spectrum. Newton achieved this by passing sunlight through a glass prism but the effect is the same, and equally enjoyable.
Interestingly, although Newton observed 7 colours when he separated white light with his glass prism, most scientists today agree that it is really only possible to discern 6 colours and that indigo cannot be distinguished from violet in the visible spectrum. Again, don’t take my word for it. Go out and look a rainbow now!!! The following relationships between colour and wavelength are often quoted:
Red —- 635-700nm
Orange —- 590-635nm
Yellow —- 560-590nm
Green —- 490-560nm
Blue —- 450-490nm
Violet —- 400-450nm
However, be very careful. Newton famously wrote that “to speak properly, the rays are not coloured”. Now, I wonder what he could have meant by that?