Colour vision 1: The varieties of colour vision

This lecture aims to give you an understanding of the basic properties of colour vision in so-called "normal" and colour-blind people. This handout summarises the main points of the lecture.

There are three broad classes of colour vision in humans. Those with so-called "normal" colour vision are called trichromats, those who we usually call "colour-blind" are called dichromats, and the very rare people who have no colour vision at all are called monochromats. It is instructive to compare these three types of colour vision.

• A trichromat needs three pairs of colour words to describe all of the colours that they can see. These word-pairs might be black-white, blue-yellow, and red-green, but they don’t have to be. For example, pink might be described as a "whitish red with a some blueness".
• A dichromat needs only two pairs of colour words to describe all the colours that they can see. These word pairs might correspond to what a trichromat would call black-white and blue-yellow.
• A monochromat needs only one pair of colour words to describe all of the colours that they can see. This word pair is probably black-white.

• A trichromat can make any colour or brightness discrimination that a dichromat can, and more besides.
• A dichromat can make any colour or brightness discrimination that a monochromat can, and more besides.
• A monochromat can make no colour discriminations and can distinguish pairs of objects on the basis of differences in brightness only.

• A trichromat can take any three coloured lights, and by mixing them in suitable proportions (by adjusting their brightnesses), can make the mixture appear identical to (ie match) any other comparison light.
• A dichromat needs to mix only two coloured lights in suitable proportions to match any other comparison light. A trichromat would probably not agree that the dichromat had made a good match.
• A monochromat needs to adjust the brightness of only one coloured light to match any other comparison light. For example, they could match a green light by adjusting the brightness of a light of any other colour. Neither trichromats nor dichromats are likely to agree that the matches that the monochromat makes are good ones.

These three kinds of colour vision form such a neat graded series that we might wonder whether colour vision has to stop with trichromats. We might envisage a tetrachromat, who needs four pairs of colour words to describe all the colours that they can see, who can make any colour discrimination that a trichromat can, and more besides, and who needs to have four lights in their colour mixtures if they are to make a match to a comparison light. We could go further, with pentachromats, hexachromats, etc. In the next lecture we will see that there is no reason why these higher degrees of colour vision couldn’t exist. In doing so, we realise that the so-called "colour-normal" (trichromats) are themselves colour blind.

More on colour mixing

The ability of people to make perfect matches to any coloured light by mixing a very restricted number of other lights (three, for a trichromat) is the most important feature of colour vision. Most of us are used to mixing coloured paints, and the results when we come to mix lights can be surprising. In particular, when we mix a green light and a red one, we obtain a yellow light; when we combine red, green and blue lights, we obtain a white light. Note that the yellowness of a red-green mixture of light is entirely a result of the way the observer's visual system processes the mixture. Physically, the two components do not interact at all.

It’s common to refer to the three lights that are mixed in a colour-mixing experiment as primaries. However, they do not have to be primary colours in the sense that we commonly use that term. The three lights can be any colours we like, provided that we cannot mix two of them to exactly match the third. In addition, we have to allow ourselves to mix negative quantities of any of our lights. Obviously, this is physically impossible, but we can achieve an equivalent result by adding one of our three lights to the comparison light that we are trying to match (this will be demonstrated in the lecture). The easiest way to avoid these complications is to use primaries that are widely separated in colour---hence the common use of red, green and blue in colour-mixing systems such as colour television.

Colour opponent phenomena

Certain phenomena suggest that the human visual system regards certain colour sensations as opposite. For example, after viewing a red light for a few minutes, we observe a green after-image; with a yellow light, the after-image is blue. In addition, notice that some colour sensations seem to be mutually exclusive: whereas we can have a reddish blue or a greenish yellow, etc, we can't have a reddish green or a yellowish blue. Some rather arcane psychophysical observations support the idea of opponent-colour processing in humans.

In this lecture we have covered some of the most basic psychological properties of human colour vision. In the next lecture we shall find that physiology provides a tidy explanation for all these properties.

Reading

I have deliberately chosen to cover all the basic psychological phenomena first before looking at the physiology. The books tend to mix the two more intimately, so it’s hard to separate the reading for this lecture and the next. However, reading about the physiology now may help you follow the next lecture.

Sekuler and Blake, Chapter 6.
Coren, Ward and Enns, Chapter 5.
Goldstein, Chapter 4.
Mollon, J.D. (1982) Colour vision and colour blindness. In Barlow and Mollon (eds) The Senses. Cambridge: Cambridge University Press.
      Sections 9.1, 9.2, 9.4, 9.8, 9.9. You may find 9.7 interesting, but the remaining sections are distinctly heavy going.
Mollon, J.D. (1982) Color Vision. Annual Review of Psychology, 33, 42-44, 49.
Mollon, J.D. (1979) The theory of colour vision. In Connolly, K. (ed) Psychology Survey No. 2 . London/Boston: Allen and Unwin.
       pp 128-150.

Alternative sources for the basics are:

Boynton, R.M. (1979) Human Color Vision . New York: Holt, Rinehart, andWinston.
Cornsweet, T.N. (1970) Visual Perception . New York: Academic Press. pp 135 ff.

Appendix: Geometric representation of colour mixtures

The following material isn't examinable, but is included because some of you may find it useful to think about colour mixing geometrically; I certainly do.

Suppose that we take three well-separated primaries, eg red, green and blue, and represent them by the corners of a triangle. Any light that we can obtain by mixing red and green will be plotted along the line joining the red and green primaries; the more red there is in the mixture, the closer the plotted point will be to the red primary, the more green there is in the mixture, the closer the plotted point will be to the green primary. If there are equal amounts of red and green, the point will be plotted halfway between the red and green primaries: this is yellow. We can apply this rule to any pair of the primaries; thus various shades of purple lie along the line joining the red and blue primaries. (Figure 1). We can extend this principle to three-colour mixtures: any light is plotted by a point within the triangle, such that the amount of any of its constituent primaries increases with the closeness of that point to the primary in question. The point in the centre of the triangle is equidistant from all primaries, and represents white. (Figure 2).

The geometric mixing rule applies to any two or three points in the triangle. Thus, suppose we take as primaries the central point (white) and the red corner. We can mix any light that lies along the line joining the two: these will be reds of varying saturation (vividness). If the mixture lies exactly on one side of the triangle, then the amount of the primary corresponding to the opposite corner of the triangle is zero.

Given three points in the triangle representing lights, the only colours that we can match with positive quantities of those lights are those colours that lie within the triangle formed by the three points. We can now see how the two provisos to our trichromacy principle arise.

Suppose that we try to match the light D by a mixture of A, B, and C (see Figure 3). D lies outside the triangle ABC: to match it we'd like to add an amount of C which takes us beyond the side AB of the triangle, which means a negative quantity of C. We can't do this with a real light. But our colour matching principle allows us to mix one of our primaries with the to-be-mixed light instead. By mixing C to D appropriately we can mix a light that lies at any chosen point along the line CD. By mixing A and B appropriately, we can obtain a light that lies anywhere along the line AB. Thus we can mix the light E in both cases.

If one primary can be matched by a mixture of the other two, it follows that it must lie on the line between the other two primaries (Figure 4). There is thus no way that we can match any light which lies off this line.

Copyright © 1998
Ben J. Craven
All rights reserved.

Last updated on Wednesday, 10/02/02, at 08:32 PM by