System for the numerical classification of colours of light and body colours. The chromaticity diagram is a two-dimensional diagram in which the colour loci of all colours and colour mixes are represented in grades of saturation ranging from the pure colour through to white, which can be numerically expressed by their xy coordinates. Colour mixes are located on a straight line drawn between the colours to be mixed; the colour of light of thermal radiators lies on the defined curvature of the Planckian curve.