Painting: Why the Shadows on White Are Blue
White as receptive of all colour; the blue of shadows and the colour of air
Headed 'Painting', this precept explains why the shadows on a white object seen in high open country appear blue. Since white is not itself a colour but is receptive of every colour, and since the surface of any opaque body takes on the colour of what faces it (Leonardo's 'fourth'), the lit white takes the colour of sun and air while the part shaded from the sun takes only the colour of the air. He adds that if such white saw neither the green of the countryside to the horizon nor the whiteness of that horizon, it would simply show the colour of the air itself. The page carries no diagram.
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Blue shadows on white in open country
Because white is not a colour but is receptive of every colour, a white object in high open country has all its shadows blue. The part that sees the sun and air takes the colour of sun and air, while the part shaded from the sun takes only the colour of the air.
White receives the colour of its surroundings
By the fourth proposition, the surface of every opaque body participates in the colour of its object. Leonardo adds that if such white saw neither the green of the countryside to the horizon nor the whiteness of the horizon, it would without doubt appear the simple colour of the air.