Colour theory was originally formulated in terms of three “primary” or “primitive” colours—red, yellow and blue, as these colours were believed capable of mixing all other colours. This colour mixing behaviour had long been known to printers, dyers and painters, but these trades preferred pure pigments to primary colour mixtures, because the mixtures were too dull (unsaturated).
According to traditional colour theory based on subtractive primary colours and the RYB colour model, which is derived from paint mixtures, yellow mixed with violet, orange mixed with blue, or red mixed with green produces an equivalent grey and are the painter’s complementary colours. These contrasts form the basis of Chevreul’s law of colour contrast: colours that appear together will be altered as if mixed with the complementary colour of the other colour. Thus, a piece of yellow fabric placed on a blue background will appear tinted orange, because orange is the complementary colour to blue.
One reason the artist’s primary colours work at all is that the imperfect pigments being used have sloped absorption curves, and thus change colour with concentration. A pigment that is pure red at high concentrations can behave more like magenta at low concentrations. This allows it to make purples that would otherwise be impossible. Likewise, a blue that is ultramarine at high concentrations appears cyan at low concentrations, allowing it to be used to mix green. Chromium red pigments can appear orange, and then yellow, as the concentration is reduced. It is even possible to mix very low concentrations of the blue mentioned and the chromium red to get a greenish colour.
The distinction between “warm” and “cool” colours has been important since at least the late 18th century. The contrast, as traced by etymologies in the Oxford English Dictionary, seems related to the observed contrast in landscape light, between the “warm” colours associated with daylight or sunset, and the “cool” colours associated with a grey or overcast day. Warm colours are often said to be hues from red through yellow, browns and tans included; cool colours are often said to be the hues from blue green through blue violet, most greys included. There is historical disagreement about the colours that anchor the polarity, but 19th-century sources put the peak contrast between red orange and greenish blue.
Colour theory has described perceptual and psychological effects to this contrast. Warm colours are said to advance or appear more active in a painting, while cool colours tend to recede; used in interior design or fashion, warm colours are said to arouse or stimulate the viewer, while cool colours calm and relax. Most of these effects, to the extent they are real, can be attributed to the higher saturation and lighter value of warm pigments in contrast to cool pigments. Thus, brown is a dark, unsaturated warm colour that few people think of as visually active or psychologically arousing.
Any colour that lacks strong chromatic content is said to be unsaturated, achromatic, near neutral, or neutral. Near neutrals include browns, tans, pastels and darker colours. Near neutrals can be of any hue or lightness. Pure achromatic, or neutral colours include black, white and all greys.
Near neutrals are obtained by mixing pure colours with white, black or grey, or by mixing two complementary colours. In colour theory, neutral colours are easily modified by adjacent more saturated colours and they appear to take on the hue complementary to the saturated colour; e.g., next to a bright red couch, a grey wall will appear distinctly greenish.
Black and white have long been known to combine “well” with almost any other colours; black decreases the apparent saturation or brightness of colours paired with it, and white shows off all hues to equal effect.