By Daniel Kernell, University of Groningen, The Netherlands.
People often talk about colours. Hence, it might seem important that they then talk about the same things. This is, however, not always the case. Colours exist only in the eyes and brains of the beholder and, to a surprising extent, different people may perceive different colours for the same object under the same circumstances. Even among individuals with the average (i.e. “normal”) kind of colour vision, such differences occur. Close to the spectral transition of blue vs. green, what is “typically green” for one person might look “blue” to another one. More drastic differences emerge, of course, between the average person and people belonging to colour vision minorities, the so-called colour blinds.
Colours are seen thanks to the different wavelength sensitivities of three classes of retinal sensor cells, the cones. The most common deviation of colour vision is the inherited red-green blindness, which is caused by deviating properties or absence of either one of the two classes of cones with a high sensitivity for the longer wavelengths of light (M and L cones). The main symptom of this deviation is that many colours which look different to the normal observer might look identical for the red-green blind. Thus, as compared to normals, less colour variation is seen by the eyes and brains of red-green blinds. However, this does not mean that it all looks grey and drab. I know this from my own eyes and brain because I happen to see the world using only two kinds of cones. I am a so-called “protanope” which means that I lack functional L cones, i.e. those with a sensitivity extending up to the very longest wavelengths of visible light.
How does the world look for the protanope? I am sorry to say that I cannot tell you this very clearly because I have seen nothing else. Some simple things may be directly measured. Many differently-named colours may look exactly the same (e.g. certain varieties of green vs. orange), and deep red colours (very long wavelengths) typically look very dark. However, I could not paint a picture showing you how it all looks for me. In this case there is a philosophically interesting problem of communication: due to my many “confusion colours” (i.e. those looking the same for me but not for you) I might paint hundreds of landscapes with colours that would all look different for you and still all look the same for me. Which one shows you what I had seen?
Also my visual world is, for me, very markedly coloured and I enjoy the colours. At the beginning of the computer age there were only monochromatic monitors; I greatly enjoyed the arrival of colour screens. Purely theoretically I know, of course, that the “colour-normal” world contains many more colours than those I see. How many more? Colour is a three-dimensional phenomenon: two colours may differ from each other in hue (i.e. the kind of colour quality labelled as red, green, blue, etc.), but also in colour saturation (i.e. relative amount of white/grey/black admixture), and light intensity. Measurements have shown that, when comparing colours directly side-by-side, an average human subject (i.e. a “normal trichromat”) may distinguish between about 150 hues, and also between at least about 100 levels of saturation and 100 levels of intensity. This means that he/she may see the differences between at least 150 x 100 x 100 = ca 1.5 million colours, For a red-green blind person of my kind (i.e. the “worst” kind) the corresponding number is, of course, smaller but it is still very large. A protanope may distinguish between only about 21 spectral hues, but still between many levels of saturation and intensity (perhaps 50 and 100 respectively; exact measurements are scarce). When multiplied by each other these smaller numbers still deliver a respectable total product of about >100 000 different colours. Thus, as was already said above, the world of the red-green blind is very far from being colourless.
Besides humans, Old World apes and monkeys are also normally trichromats, i.e. they have three kinds of cones with different wavelength sensitivities. However, most other kinds of mammals are dichromats, having only two kinds of cones. With regard to the perceived coloration of the world, my minority kind of primate is much more characteristic for mammals in general than is the case for the average human variety. Thus, if you would like to train mammalian animal friends for using coloured objects or signals, it might occasionally even be useful to consult a “specialist” on similar kinds of colour vision: a human red-green blind dichromat.