...in the parts interposed between the lights and the shadows, the parts of those bodies are in the first degree of distinctness.

Discourse on painting.
Perspective, as it extends into painting, is divided into three principal parts. The first is of the diminution that the sizes of bodies undergo at various distances; the second part is that which treats of the diminution of the colours of such bodies; the third is that which diminishes the distinctness of the figures and boundaries that those bodies have at various distances.

On painting. Book A, folio 1.
The blue of the air is a colour compounded of light and darkness: by the light I mean that caused by the air illuminated in the particles of moisture infused within it; by the darkness I mean the pure air, which is not divided into atoms, that is, into particles of moisture, on which the sun's rays might strike. And an example of this is seen in the air that lies between the eye and the shadowy mountains — shadowy because of the shade cast by the great abundance of trees found upon them, or shadowy in that part which is not struck by the sun's rays — which air becomes blue, and does not become blue in its luminous part, and worse still in the part covered with snow.

Precept.
Among things equally dark and at equal distance, that one will show itself to be darker which ends against a whiter field, and the converse likewise.

Precept.