In many styles and periods of painting, the functions of colour are primarily decorative and descriptive, often serving merely to reinforce the expression of an idea or subject communicated essentially in terms of line and tone. In much of modern painting, however, the full-spectrum range of pigments available has allowed colour to be the primary expressive element.
The principal dimensions of colour in painting are the variables or attributes of hue, tone, and intensity. Red, yellow, and blue are the basic hues from which all others on the chromatic scale can be made by mixtures. These three opaque hues are the subtractive pigment primaries and should not be confused with the behaviour of the additive triads and mixtures of transparent, coloured light. Mixtures of primary pairs produce the secondary hues of orange, violet, and green. By increasing the amount of one primary in each of these mixtures, the tertiary colours of yellow-orange, orange-red, red-violet, violet-blue, blue-green, and green-yellow, respectively, are made. The primary colours, with their basic secondary and tertiary mixtures, can be usefully notated as the 12 segments of a circle. The secondary and tertiary colour segments between a pair of parent primaries can then be seen to share a harmonious family relationship with one another—the yellow-orange, orange, and orange-red hues that lie between yellow and red, for example.
Local hues are the inherent and associative colours of things. In everyday life, familiar things are described by particular colours, and these often are identified by reference to familiar things; the green of grass and the grass green of paint, for instance. Although, as the Impressionists demonstrated, the inherent colours of forms in the real world are usually changed by effects of light and atmosphere, many of the great “primitive” and classical styles of representational painting are expressed in terms of local hues.

Tone is a colour’s relative degree, or value, of lightness or darkness. The tonal pattern of a painting is shown in a monochrome reproduction. A painting dominated by dark colours, such as a Rembrandt, is in a low tonal key, while one painted in the pale range of a late Claude Monet is said to be high keyed. The tonal range of pigments is too narrow for the painter to be able to match the brightest lights and deepest darks of nature. Therefore, in order to express effects of illumination and dense shadow, he must lower the overall tonal key of his design, thus intensifying the brightness value of his lightest pigment colours.