Primary Colours

They are colours that can be mixed in varying amounts to produce a Gamut of Colours I like.

Art education materials commonly use red, yellow and blue as the primary colours, sometimes suggesting they can mix all colours. No real set of real colorants or lights can mix all possible colours however.

It is because in the chromaticity diagram, the real colour space is in a horseshoe shape, and the colour gamuts are in a convex shape. This is because the primary colours merge to the white point in the middle, so any addition of colours corresponds to a colour in the interior of the triangle formed by the primary colour coordinates.

The primary colours are defined arbitrarily, in the sense that no set of primary colours can be the canonical set. Colour space primaries are chosen depending on the Colour Space, which in turn is defined according to the subjective preferences, as well as practical factors such as cost, stability and availability.