At this point the physics of light becomes less important than simply positing some useful models of colour organization. The colour wheel introduced above must be familiar to most readers, but if it is given three “spokes” or diameters then we have three axes of the complementaries, linking each primary colour with the secondary that results from the other two. The physiological basis for these complementary colours lies in the spectral range that each reflects, as mentioned previously. When we look at a surface strongly coloured by a primary the receptors in our retinae react to the full range of the spectral colour reflected, but these receptors are rapidly exhausted, leaving active only those receptors which respond to the other part of the spectrum. Thus we see an “afterimage” of the complementary colour.
As the Impressionists in 19th century France both attempted to respond more directly to the raw data of their eyes and also to accommodate the increasing sophistication of colour theory, they placed increasing reliance on complementary colour transitions. It is almost too well known to state that they recorded yellow sunlight producing violet shadows, green foliage making tree trunks look red, and blue skies dotted with orangey clouds. But if, thanks in part to the writings of Chevreul and Ogden Rood , impressionists and neo-impressionists had become more conscious of complementarity, it does not follow that previous artists were not informally aware of this principle in their handling of colour. It had long been appreciated that delicately determined greys could be made by directly mixing colours which in fact were complementaries. Painters such as Veronese, Rubens and Delacroix had in different ways introduced green semitones into passages of pink flesh. Claude or Cuyp landscapes commonly show amber light throwing blue shadows across clouds. But clearly the tonal structure of these artists’ works functioned robustly and independently of their constituent colours. This brings us to more complex models of colour construal, and also some definition of terms.
As stated before, the range of artist’s colours extends far beyond the spectral ones. Most conspicuously, it includes white, reflecting the whole spectrum, and black, absorbing all of it. It also includes browns, dull greens, maroons, blackish blues and so on. How can these be related to the colour wheel and its three pairs of complementaries? The solution lies in a variety of models which take instead the circle of continuously gradated colour, (of which the colour wheel is a sixfold reduction), as a two dimensional plane, so to speak, but add a third dimension running above and below it. Above it runs the scales of hues: the scale of colours that result from successive additions of white to every minutely different colour on the circle, until white is reached. Below it runs the scale of tints or shades: the scale that results from increasing additions of black to each colour on the circle, until black is reached. If we interpret the pure white and pure black extremities as respectively, the north and south poles of a sphere, of which the colour circle is the equator line, then we have the convenient colour-sphere first proposed by the German artist Runge in 1810. The equator line can be further regarded as the rim of the colour circle where colours are at their most intense or most saturated, and we can determine our position on the circle according to the chroma of these colours. Bit if we follow each radius from each chromatic position to the centre of the sphere then we can construct a scale of diminishing saturation, that is, the scale from each maximally intense colour, through successive additions of grey which is of the same tonal weight or value, until absolute grey is reached at the centre. It is along this scale of diminishing saturation of a given colour that we can assess the quality of an individual paint as we obtain it: the greater the proportion of filler, the less saturated the colour of the paint, the “greyer” it appears to us.
This model usefully incorporates locations for the artist’s colours mentioned above; it is clear that browns and some reds are interpretable as particular tints of fully saturated oranges and reds. The need to provide classificatory systems to specify colours in industrial dyeing, printing and pigment manufacture has in the last century produced the complex models of coded colours devised by Pope, Ostwald, or most in use nowadays, Munsell. But these systems are most useful in industry, and the assistance that models like Runge’s provides, is in leading the artist to formulate a mental geometry of colour in the same way as we presuppose a physical geometry in our perception of form. They are merely aids to understanding, and do not represent structures “out there”.
What such systems do lead us to appreciate is how, particularly in European painting from the 15th to the 19th centuries discussion of colour is inseparable from that of tonal weight or value. They also show how such a use of different tonalities is “implicitly complementary”. For example, a rich brown, as a tint of orange, will set off a hue of blue. The exploitation of the mutually intensifying effects of juxtaposed complementary colours, is simply one strategy in the tonal construction of a painting. The works of Velasquez, Poussin and much of Rubens indicate that colorific variety is essentially ordered by what French theorists dubbed “the values.”