Painters who are not colourists make illuminations and not paintings. Painting, properly speaking--unless one wants to produce a monochrome--implies the idea of colour as one of its fundamental elements, together with chiaroscuro, proportion, and perspective. Proportion applies to sculpture as to painting. Perspective determines the outline; chiaroscuro produces relief by the arrangement of shadow and light in relation to the background; colour gives the appearance of life, &c.
The sculptor does not begin his work with an outline; he builds up with his material a likeness of the object which, rough at first, establishes from the beginning the essential conditions of relief and solidity.
Colourists, being those who unite all the qualities of painting, must, in a single process and at first setting to work, secure the conditions peculiar and essential to their art.
They have to mass with colour, as the sculptor with clay, marble, or stone; their sketch, like the sculptor's, must show proportion, perspective, effect, and colour.
Outline is as ideal and conventional in painting as in sculpture; it should result naturally from the good arrangement of the essential parts. The combined preparation of effect which implies perspective and colour will approach more or less the actual aspect of things, according to the degree of the painter's skill; but this foundation will contain potentially everything included in the final result.
I believe colour to be a quite indispensable quality in the _highest_ art, and that no picture ever belonged to the highest order without it; while many, by possessing it--as the works of Titian--are raised certainly into the highest _class_, though not to the very highest grade of that class, in spite of the limited degree of their other great qualities. Perhaps the _only_ exception which I should be inclined to admit exists in the works of Hogarth, to which I should never dare to assign any but the very highest place, though their colour is certainly not a prominent feature in them. I must add, however, that Hogarth's colour is seldom other than pleasing to myself, and that for my own part I should almost call him a colourist, though not aiming at colour. On the other hand, there are men who, merely on account of bad colour, prevent me from thoroughly enjoying their works, though full of other qualities. For instance, Wilkie or Delaroche (in nearly all his works, though the Hémicycle is fine in colour). From Wilkie I would at any time prefer a thoroughly fine engraving--though of course he is in no respect even within hail of Hogarth. Colour is the physiognomy of a picture; and, like the shape of the human forehead, it cannot be perfectly beautiful without proving goodness and greatness. Other qualities are in its life exercised; but this is the body of its life, by which we know and love it at first sight.