There are many reasons why I don’t like colour wheels of the type shown below:
The first reason is because it perpetuates the myth that the subtractive primaries are red, yellow and blue whereas the fact is that red, yellow and blue produces a rather small gamut of colours. It is certainly not the best choice of subtractive primaries though it is taught as dogma in many art and design schools and throughout children’s education. The problem is that whenever two colours are mixed together there is saturation loss; that is, the resultant mixture ends up being more desaturated than the two components were. The saturation loss is greatest when mixing colours on the opposite side of the colour circle where the resultant mixture can be almost grey. However, for certain choices of primaries, the saturation loss is greater than for others. If red, yellow and blue are used as the primaries then of course it is possible to generate any other hue. However, there is significant saturation loss and the above colour wheel gives completely the wrong impression. It suggests that mixing blue and yellow together, for example, results in a really bright vivid green.
The reality of pigment mixing is much more like the triangular colour wheel shown below:
In the above diagram it can be seen that mixing together yellow and blue results in a really muddy dark green. The purple resulting from mixing blue and red is almost black!! Now it is possible to mix together a blue and a yellow to get a better green. For example, mixing a greenish blue with a yellow will give a much more vivid green. Mixing a bluish red with a greenish blue will result in a lovely purple. We have a name for a greenish blue and a blueish red – we call them cyan and magenta. A much better colour gamut is obtained if we start with the primaries, cyan, magenta and yellow.
Footnote: Some people may look at the triangular colour wheel and think that the reason the colours are dull is that the red, yellow, and blue primaries used are not ‘pure’ enough. Nothing could be farther from the truth. If it was possible to make really vivid and bright red and blue pigments then the resultant colour gamut would be even smaller. Fundamentally, red, yellow and blue just don’t make good subtractive primaries.
I sometimes begin a series of student lectures on colour with the question – how many colours are there? At least one student always answers: three! In fact, this week in my lecture when I asked this question the first three or four answers were all three.
I can see where the idea of three comes from since the number three is ubiquitous in colour. We have three different classes of cones in the retina of our eyeballs – each with maximum sensitivity at a different wavelength. As a direct consequence of this trichromacy we use colour monitors with three primaries (RGB), colour printers with three primaries (CMY – ok, sometimes black as well but there’s a good reason for that), and there is a misconception that there are three primary colours from whose mixtures it is possible to make every other colour – see http://colourware.wordpress.com/2009/07/08/what-is-a-colour-primary/
I think that the number of colours that we can see is about 10 million; maybe less, but certainly millions. However, there are arguments that the true number may be much greater than this. See, for example, Mark Fairchild’s article – http://www.cis.rit.edu/fairchild/WhyIsColor/files/ExamplePage.pdf.
However, even the people thinking about colour mixing and three primaries must surely be aware that they have seen more than three colours. Indeed, were probably wearing more than three colours! So why do they respond with three? Well, it could be that they misunderstand the question and think I am asking about primaries (perhaps because they think the real question I am asking is too hard and nobody in their right mind would ask it). Or it could be that they equate the word colour with physical colorants. One of the most interesting – but also frustrating – things in field is that even the name of the field – colour – means different things to different people. Is colour something physical? Is it something you experience? Or is it simply whether something is red, yellow or blue etc; in other words, another term for what I would call hue?
This probably explains why we find the following text on this webpage – http://english.kompas.com/read/xml/2009/10/29/06125368/The.Shrimps.That.Can.See.in.Twelve.Colors:
A juvenile Mantis shrimp. These shrimps have the most complex vision systems known to science. Special light-sensitive cells allow them to distinguish between different types of polarized light, and they can see 12 colors (compared to three for humans) ranging into the near-ultra violet to infra-red parts of the electromagnetic spectrum.
It literally says that shrimps see 12 colours whereas, elsewhere on the page, it says that humans see 3 colours. Despite this irritating lack of precision in the writing the article is quite interesting and describes the surprisingly complex nature of shrimp colour vision.