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.
_Delacroix._