Primary colours, and why there are several sets

Why there are several sets

Primaries are a choice, not a fact of nature. Any three colours that sit far apart will mix the colours that lie between them, and different jobs need different starting points. A screen starts from black and adds light, so it uses red, green and blue. A printer starts from white paper and takes light away, so it uses cyan, magenta and yellow, plus black. The eye has three kinds of colour receptor, which is why three primaries are usually enough. Even so, no set of three real primaries can make every colour a person can see.

Red, yellow and blue

The red, yellow and blue set comes from painters' practice in Europe in the 1600s and 1700s, and the colour wheels of the next two centuries fixed it in teaching. It worked because those were the strongest pigments to hand. Its weakness shows when you mix. Red and blue paint often give a dull purple, and blue and yellow a muted green. Magenta and cyan make cleaner violets and greens, which is why printers moved to them. Many painters get round the problem by keeping a warm and a cool version of each primary on the palette.

The four colours that look pure

A third idea of primary comes from how colours look, not how they mix. In the 1870s the German physiologist Ewald Hering argued that people see four pure hues: red, yellow, green and blue. None of them looks like a blend of the others. Orange looks like red and yellow together, but nobody sees a reddish green. These four are often called the unique hues. With black and white they form the basis of the Natural Colour System used in Scandinavian design, and they match the way the visual system codes colour after light has left the eye's receptors.

Colours on this page

Sources and further reading

• Primary color on Wikipedia
• Handprint: colour-making attributes and the primary colours
• Stanford Computer Graphics Laboratory: Additive versus subtractive colour mixing
• Webexhibits, Causes of Color: Colour mixing