Why no body shows its true colour, save blue against a mountain sky
Strong versus weak light, and the single case in which natural colour survives
Leonardo notes that the difference between lights and their shadows is far greater in bodies set in powerful light than in those in dark places, and that the part of a shadowy body facing the strongest light shows most luminous. Citing his Book A (chapters 19 and 21), he argues that a body's shadow grows in proportion as the darkness it faces exceeds the splendour that lights it, and that no body ever shows itself entirely of its natural colour. The single exception, he says, is blue laid flat against the sky on a very high mountain where nothing else is in view and the sun is hidden by low cloud — though he then half-retracts, since rosy colour too gains in beauty.
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Strong light widens the gap between light and shadow
The difference between lights and their shadows is much greater in bodies placed in powerful lights than in those set in dark places. The part of a shadowy body that shows most luminous is that illuminated by the more powerful light, and its shadow grows as the darkness it faces exceeds the splendour that lights it (Book A, ch. 19).
No body ever shows entirely its natural colour
No body will ever show itself entirely of its own natural colour. This happens for two causes: the interposition of the medium enclosed between object and eye, or the colour of the light that illuminates the body (Book A, ch. 21).
The one case: blue against the sky on a high mountain
A body would show its natural colour only if lit by a colourless light and seeing no other object than that light. Leonardo says this can be seen only in blue laid flat toward the sky on a very high mountain, where nothing else is visible, the sun occluded by low clouds at its dying, and the plain the colour of the air; yet he then retracts, since rosy colour also grows in beauty.