the white . . . and the black

?[make] the white in your picture precious, and the black conspicuous.

I say, first, the white precious. I do not mean merely glittering or brilliant: it is easy to scratch white sea-gulls out of black clouds, and dot clumsy foliage with chalky dew; but when white is well managed, it ought to be strangely delicious,?tender as well as bright,?like inlaid mother of pearl, or white roses washed in milk. The eye ought to seek it for rest, brilliant though it may be; and to feel it as a space of strange, heavely paleness in the midst of the flushing of the colours. This effect you can only reach by general depth of middle tint, by absolutely refusing to allow any white to exist except where you need it, and by keeping the white itself subdued by grey, except at a few points of chief lustre.

Secondly, you must make the black conspicuous. However small a point of black may be, it ought to catch the eye, otherwise your work is too heavy in the shadow. All the ordinary shadows should be of some colour,?never black, nor approaching black, they should be evidently and always of a luminous nature, and the black should look strange among them; never occuring except in a black object, or in small points indicative of intense shade in the very centre of masses of shadow. Shadows of absolutely negative grey, however, may be beautifully used with white, or with gold; but still through the black thus, in subdued strength, becomes spacious, it should always be conspicuous; the spectator should notice this grey neutrality with some wonder, and enjoy, all the more intensely on account of it, the gold colour and the white which it relieves. Of all the great colourists Velasquez is the greatest master of the black chords. His black is more precious than most other people?s crimson.?

?John Ruskin, from The Elements of Drawing, 1857.