How Grounds Change a Color: The Lesson of Falling Snow
White looks whiter on a dark ground; snow, flesh and pale tones are judged by the grounds that surround them.
This leaf of the Treatise on Painting develops Leonardo's rule of simultaneous contrast: a white thing looks whiter the darker its ground and darker the whiter its ground. He proves it by falling snow, which looks dark against the bright sky yet brilliantly white seen against the shadow within an open window, and adds that near snow seems swift and continuous, remote snow slow and broken. Further chapters show that flesh-tone looks pale on red and ruddy on yellow, so colors are judged to be what they are not by their surroundings, and open the great question of grounds for painted bodies.
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White appears whiter on a dark ground: the snow proof
A white thing shows whiter the darker its ground and darker the whiter its ground. Falling snow looks dark against the bright air but most white against the shadow seen through an open window, near snow appearing swift and like continuous white cords, remote snow slow and discontinuous.
Grounds alter the apparent color of figures
Of things equally bright, the one on the whiter ground looks less bright, and the one in a darker space looks whiter; flesh-color appears pale on a red ground and ruddy on a yellow one, so colors are judged to be what they are not by the grounds that surround them.
The dignity of grounds for painted bodies
Leonardo calls the discourse of grounds a matter of the greatest importance, since opaque bodies clothed in shadow and light must stand out upon them, their lit parts wanting dark grounds and their dark parts light ones, as the argument continues on the next leaf.