The principles connected with colouring should be understood if one desires to produce the most pleasing and harmonious effects in painting. The three colours, red, yellow and blue, with the white of the canvas, are equal in theory to all the requirements of art in its true relationship with colour. Red, yellow and blue are called primary colours, that is, we cannot produce these colours from the combination of any others. Orange, purple and green are called secondary colours, and are produced by the combination of the primary colours. By the mixture of red and yellow we obtain orange, from red and blue, purple, from yellow and blue, green. The tertiary colours, green, gray and brown, are produced by the mixture of the secondary colours. From orange and purple we obtain brown, from orange and green, green, and from purple and green, gray.
The three primary colours must always be present in a visual image to produce harmony. Colours are divided into what are called warm and cold colours, the yellow and red being termed warm, and the blue cold. Yellow and red produce light and warmth, and it is impossible to produce coolness without the use of blue. In painting we use the three terms, light, shade and colour, because they best express the qualities of colour. Light is expressed by yellow, shade by blue, and colour by red. While red is particularly designated as colour, we must not forget the claims of yellow and blue, as they, together with red, complete the primary scale of colours. It is by placing these different colors in juxtaposition that we produce the proper qualities existing in each of the other colours. It is impossible to produce the effect of warmth by red and yellow unless we use the blue in connection with them. It is this filling up, or completing the primary scale of colours, that gives the term complementary, so often used in speaking of colours. Thus red is said to be complementary to green, as green contains the other two colours of the primary scale—blue and yellow. Blue is complementary to orange, as orange contains red and yellow. Yellow is complementary to purple, as purple contains blue and red. The principle of using the complementary colour is of the utmost importance in painting, or the use of colour by any path toward the aesthetic and it is, in my opinion too, on this principle that the harmony of colour is based. When a painting is produced that has the colours red, yellow and blue properly balanced, a pleasing and harmonious effect is attained; but if these colours are not used in their proper relations, there is a discord, and the work is not satisfactory.
These rules must be borne in mind by people when colouring, whether on uses oil or water colours, it is all relative. One of the most common errors of anyone using colour is to overlook the red in landscape. Thus trees are too green, and the grass is insufferably green: the complementary colour, red, has been left out.