Why is colour theory useful?
One reason the artist’s primary colors work at all is that the imperfect pigments being used have sloped absorption curves, and thus change color 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 color. This works much better with oil colors than it does with watercolors and dyes.
So the old primaries depend on sloped absorption curves and pigment leakages to work, while newer scientifically derived ones depend solely on controlling the amount of absorption in certain parts of the spectrum.
Another reason the correct primary colours were not used by early artists is that they were not available as durable pigments. Modern methods in chemistry were needed to produce them.
Warm vs. cool colours.
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.
Contrast the traditional warm–cool association of colour with the colour temperature of a theoretical radiating black body, where the association of colour with temperature is reversed. For instance, the hottest stars radiate blue light (i.e., with shorter wavelength and higher frequency), and the coolest radiate red.
This contrast is further seen in the psychological associations of colours with the Relativistic Doppler effect seen in astronomical objects. Traditional psychological associations, where warm colours are associated with advancing objects and cool colours with receding objects, are directly opposite those seen in astrophysics, where stars or galaxies moving towards our viewpoint from Earth are blue shifted (advancing) and stars or galaxies moving away from Earth are redshifted (receding).
Split primary colours
In painting and other visual arts, two-dimensional colour wheels or three-dimensional colour solids are used as tools to teach beginners the essential relationships between colours. The organization of colours in a particular colour model depends on the purpose of that model: some models show relationships based on human colour perception, whereas others are based on the colour mixing properties of a particular medium such as a computer display or set of paints.
This system is still popular among contemporary painters, as it is basically a simplified version of Newton’s geometrical rule that colours closer together on the hue circle will produce more vibrant mixtures. However, with the range of contemporary paints available, many artists simply add more paints to their palette as desired for a variety of practical reasons. For example, they may add a scarlet, purple and/or green paint to expand the mixable gamut; and they include one or more dark colours (especially “earth” colours such as yellow ochre or burnt sienna) simply because they are convenient to have premixed. Printers commonly augment a CMYK palette with spot ink colours.
It has been suggested that “Colours seen together to produce a pleasing affective response are said to be in harmony”. However, colour harmony is a complex notion because human responses to colour are both affective and cognitive, involving emotional response and judgment. Hence, our responses to colour and the notion of colour harmony is open to the influence of a range of different factors. These factors include individual differences (such as age, gender, personal preference, affective state, etc.) as well as cultural, sub-cultural and socially-based differences which gives rise to conditioning and learned responses about colour. In addition, context always has an influence on responses about colour and the notion of colour harmony, and this concept is also influenced by temporal factors (such as changing trends) and perceptual factors (such as simultaneous contrast) which may impinge on human response to colour.