PART 3. SOME BASICS OF COLOUR VISION

3.1 Trichromacy, Opponency and Constancy

All art students are aware of the concepts of the colour wheel, the spectrum, and the primary colours. They may even know just enough to get into debates over whether the "real" primary colours are the red, yellow and blue of the conventional artists' colour wheel, or the yellow, magenta and cyan printer's primaries, or perhaps the three optimal primaries for mixing coloured lights - red, green and blue. But few have reflected on the apparent inconsistencies that lurk within this widely held knowledge. For example, few have wondered why there are three primary colours. If the visible spectrum consists of a continuous range of wavelengths, why should just three colours be special? Why not five? And why for that matter do colours form a circle, when the spectrum does not? Though fundamental to the subject, these questions do not normally occur to us, perhaps because we internalize our knowledge about colour at an early and uncritical age. To answer them we need to understand some basics of colour vision.

In broad outline, colour vision is currently understood to involve three major processes:

1. Trichromatic input: information is recorded by the responses of the L,M and S cone cells in the retina.

2. Opponent output: responses from the L,M and S cones are compared with each other in the retina and the lateral geniculate nucleus in a process called cone opponency. Cone oppoenecy should not be confused with hue opponency, which occurs at a later and poorly understood stage, in which visual information emerges as perceptions or "signals" of yellowness vs blueness and redness vs greenness, plus total brightness.

3. Processing for colour constancy: information from throughout the visual field is rapidly, automatically and seemingly effortlessly analysed and resolved into an interpretation of object properties (seen as their hue, value and chroma) and lighting properties (seen as the hue, brightness and saturation of the illumination) and atmospheric properties.

We will review these processes briefly in the succeeding pages, not only to answer the questions raised in the first paragraph, but also to gain the background we need in order to discuss the dimensions of colour in detail. For a much more comprehensive online account of current understanding of all aspects of colour vision, the Webvision site of the University of Utah is strongly recommended. For a short video outlining these processes in the simplest possible terms, please see my entry in the 2014 Flame Challenge (Section 11.4).

Unsurprisingly these processes are entirely ignored in "traditional" colour theory, but unfortunately they are also quite misleadingly treated in the majority of popular scientific explanations of colour. Such explanations very generally follow a standard pattern that includes a version of the trichromatic theory of input from three cone types, but completely omits both cone and hue opponency. Instead, these explanations talk as if colours such as red and yellow exist within the spectrum itself , and often as if individual cone types "detect" individual colours existing in the spectrum (namely red, green and blue). These misunderstandings have in turn generated various secondary misunderstandings about colour, for example that since "real" yellow exists in the wavelengths of the spectrum that look yellow, the yellow of a computer screen, which contains few or no such wavelengths, is merely the "illusion" of yellow, or that magenta alone is not a "real" colour, because it does not "exist" in the spectrum. Two YouTube videos that have had the effect of propagating these and other misunderstandings about colour, Michael Stevens' This is Not Yellow and Steve Mould's The Mystery of Magenta, are examined in detail in Section 11.5.

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