Ruskin’s instructions to his students: 

Suppose you have to paint the Narcissus of the Alps. First, you must outline its six petals, its central cup, and its bulbed stalk, accurately, in the position you desire. Then you must paint the cup of the yellow which is its yellow, and the stalk of the green which is its green, and the white petals of creamy white, not milky white. Lastly, you must modify these colours so as to make the cup look hollow and the petals bent; but, whatever shade you add must never destroy the impression, which is the first a child would receive from the flower, of its being a yellow, white and green thing, with scarcely any shade in it. And I wish you for some time to aim exclusively at getting the power of seeing every object as a coloured space. Thus for instance, I am sitting, as I write, opposite the fireplace of the old room which I have written much in, and in which, as it chances, after this is finished, I shall write no more. Its worn paper is pale green; the chimney-piece is of white marble; the poker is gray; the grate black; the footstool beside the fender of a deep green. A chair stands in front of it, of brown mahogany, and above that is Turner’s Lake of Geneva, mostly blue. Now these pale green, deep green, white, black, gray, brown and blue spaces, are all just as distinct as the pattern on an inlaid Florentine table. I want you to see everything first so, and represent it so. The shading is quite a subsequent and secondary business. If you never shaded at all, but could outline perfectly, and paint things of their real colours, you would be able to convey a great deal of precious knowledge to any one looking at your drawing; but, with false outline and colour, the finest shading is of no use.