And, though of course you should always give as much form to your subject as your attention to its colour will admit of, remember that the whole value of what you are about depends, in a coloured sketch, on the colour merely. If the colour is wrong everything is wrong: just as if you are singing, and sing false notes, it does not matter how true the words are. If you sing at all, you must sing sweetly and if you colour at all, you must colour rightly. Give up all the form rather than the slightest part of the colour… Never mind though your houses are all tumbling down – though your clouds are mere blots, and your trees mere knobs, and your sun and moon like crooked sixpences – so only that trees, clouds, houses, and sun or moon, are of the right colours.
Against one thing, however, I must steadily caution you. All kinds of colour are equally illegitimate, if you think they will allow you to alter at your pleasure, or blunder at your ease. There is no vehicle or method of colour which admits of alteration or repentance; you must be right at once, or never; and you might as well hope to catch a rifle bullet in your hand, and put it straight, when it was going wrong as to recover a tint once spoiled.
In filling up your work, try to educate your eye to perceive these differences of hue without the help of the cardboard, and lay them deliberately, like a mosaic-worker, as separate colours, preparing each carefully on your palette, and lay ing it as if it were a patch of coloured cloth, cut, out to be fitted neatly by its edge to the next patch; so that the fault of your work may be, not a slurred or misty look, but a patched bed cover look, as if it had all been cut out with scissors.
In distant effects of rich subject, wood, or rippled water, or broken clouds much may be done by touches or crumbling dashes of rather dry colour, with other colours afterwards put cunningly into the interstices. The more you practise this, when the subject evidently calls, for it the more your eye will enjoy the higher qualities of colour. The process is, in fact, the carrying out of the principle of separate colours to the utmost possible refinement; using atoms of colour in juxtaposition instead of large spaces.
[John Ruskin. The Elements of Drawing. 1857]