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:

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.