How transparent colours change over what lies beneath
Glazes, and the part of each hue that shows most beautiful — black in shadow, white in light
A clean text page of the Treatise on Painting on the behaviour of colour. Leonardo explains that a transparent colour laid over another produces a third, mixed hue, illustrated by chimney smoke that looks blue against the black flue and grayish or reddish against the blue sky. He tabulates glazes — purple over blue giving violet, blue over yellow giving green, saffron over white giving yellow, light over dark giving blue. A second chapter asks in which of its parts each colour is most beautiful, answering that black shows best in shadow, white in light, blue, green and tawny in the middle shadows, yellow and red in the lights, gold in reflections; a further chapter heading begins at the foot of the page.
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Glazing: a transparent colour changes over what lies beneath
A transparent colour laid over another is altered by it, forming a mixed colour unlike either of the simple hues that compose it. Leonardo gives a table of glazes: purple over blue yields violet, blue over yellow yields green, and saffron over white yields yellow. The effect is strongest when the underlying and overlying tones are most excellent.
Light over darkness makes blue
The mixing of transparent tones is seen in the smoke rising from chimneys, which looks blue when set against the black of the flue but grayish or reddish when it rises against the blue of the air. Leonardo generalises the observation into a rule: light passing over darkness produces blue, the more beautiful the more excellent the light and the dark. This ties the painter's glazes to a physical account of how bright and dark media combine.
Where each colour shows most beautiful
Each colour is asked to show its beauty in a different part of itself — in its lustre, its light, its middle shadows, its darks, or in transparency. Leonardo's answer is a table: black is most beautiful in the shadows, white in the light, blue, green and tawny in the middle shadows, yellow and red in the lights, gold in the reflections, and lake in the middle shadows. A following chapter heading, on why unlustrous colours are more beautiful in their lit parts, begins at the foot of the page.