Johann Heinrich Lambert has already emphasised that the colours of light and the colours of materials behave in a different way when mixed. The sketches emphasise the three parts on which Sowerby's theory rests and express the stabilising continuity which can exist between them. Sowerby's text describes the optical mixtures which result when narrow and tightly packed strips of primary colour are applied to paper. It was to be another few decades, however, before the difference between colours was correctly understood, enabling a more precise distinction between colour mixtures.
Coloured light is mixed additively, to use the modern term.
The additive mixture of red and green will, for example, result in yellow. Violet-blue together with green will give cyan. Two colours which additively neutralise each other and combine to form white are called complementary colours. Coloured pigments have an effect that is very different to coloured light. The subtractive mixture of a yellow and a violet-blue pigment will not result in white, but black.