A Treatise on Painting, by Leonardo da Vinci
Senex and Taylor, London
If you wou'd have the Neighbourhood of one Colour, give a Grace to another, imitate Nature, and do that with your Pencil which the Rays of the Sun do upon a Cloud, in forming a Rainbow; where the Colours fall sweetly into one another, without any stiffness appearing in their extremes.
Observe further, the following Remarks relating to Colours. 1. In representing a deep darkness, be sure you oppose to it a strong White; and to set off a White with the greater force and Lustre, let there be a deep dark opposed to it. 2. Red will appear more vivid near a pale Yellow, than near a Violet. 3. You must distinguish between Colours, which set one another off with a greater force and brightness, and those which only add a Grace to each other; thus Green gives a Grace to Red, and at the same time takes it from Blue. Lastly a pale Yellow or a White, matches very ill with an Azure; the Union of these Colours as well of some others hereafter mentioned, being of mutual disservice.