“…in regard to the lights: in them the colors may be loaded as much as may be thought requisite. They have substance: it is necessary, however, to keep them pure. This is effected by laying each tint in its place, and the various tints next to each other, so that, by a slight blending with the brush, they may be softened by passing one into the other without stirring them much. Afterwards you may return to this preparation, and give to it those decided touches which are always the distinctive marks of great masters.”—Rubens
Van Mander, whose precepts are antecedent to the influence of Rubens, comments on the coloring of flesh in the highlights: “Let not your flesh color freeze; let it not be too cold or purple, for a carnation which approaches the whiteness of linen cannot bloom with the signs of life. But vermilion makes it glow with a more fleshy hue. Endeavour to produce this warmth . . . . In painting peasants, shepherds, and mariners, spare not yellow ochre with your vermilion. . . . Be careful not to light up the flesh tints in either sex with too much white; no pure white is visible in the living subject.”
The Flemish recommended that one not use black or white in shadows, as they deaden the quality, eliminating the desired glow of transparency which makes proper use of the white ground. Rubens stated, “be careful not to let white insinuate itself into (your shadows); it is the poison of a picture except in the lights; if white be once allowed to dull the perfect transparency and golden warmth of your shadows, your coloring will no longer be glowing, but heavy and gray.”
“The Flemish…were careful to preserve transparency as much as possible in the darks; for, whatever be the nature of the color, internal light still exhibits its maximum of warmth.”
Shadows should depict a uniformity of tone, a—“simple unity of shade, as all were from a single palette spread.”—Du Fresnoy
Rubens general transparent shade fulfilled this rule.
“The purest color in an opaque state and superficially light only, is less brilliant than the foulest mixture through which light shines.” In Flemish works a thinner medium was used for the lights than the darks, so that, even in the lights, the white ground affected the color. “If those painters erred, it was in sometimes too literally carrying out this principle. Their lights are always transparent (mere white excepted) and their shadows sometimes want depth. This is in accordance with the effect of glass-staining, in which transparency may cease with darkness but never with light.
The superior method of Rubens consisted in preserving transparency chiefly in his darks, and in contrasting their lucid depth with solid lights.” Lights become more translucent with age, but darks deepen and lose detail and brilliance. So emphasize the translucence of shadows from the start.
And though Rubens warned against whites that cause opacity in the shadows, a dry white preparation underneath the rich darks is by no means prohibited; indeed it existed in the white ground. So if one creates form with white in the shadows, it can then be glazed with transparent color for a similar effect as that which makes use of the white ground. This was done by the Italians, who sometimes began a work in white and black, glazing them to the highest degree of warmth. Such was the practice of Tintoretto, and of Titian who, after painting his grisaille in lead white and carbon black, layered it with red and yellow ochre and cinnabar (vermilion) plus lead white in the lights. (see Reynolds observations below) And even Rubens used white to create reflected lights, so that his instructions must be understood within the particular method in which they were employed.