The Highlights of Leaves: Blue Sky, Yellow Leaves, Green Luster
How reflected sky and leaf color combine into the sheen of foliage
In three linked notes Leonardo analyzes the highlights on leaves as reflections of the sky. On dark leaves the polished surface mirrors the blue of the air, and light over dark composes a bluish sheen; on yellowish-green leaves the same blue sky, seen in yellow, appears green, since blue and yellow mixed make a beautiful green, so their luster is yellow-green. He closes with trees lit by both sun and air: the air-lit side takes on blue, while the side the eye sees struck by the sun is lustrous.
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Dark leaves take a blue sheen from the mirrored sky
The highlights of darker leaves partake most of the color of the air reflected in them, because bright light over the leaf's own dark composes blue. That brightness comes from the blue of the air mirrored on the polished leaf surface, which strengthens the blue that light tends to generate against dark things.
Sky reflected in a yellow leaf reads as green luster
Since everything seen in a mirror takes the color of that mirror, yellowish-green leaves make no bluish luster: the blue of the air mirrored in the yellow of the leaf appears green, blue and yellow together composing a most beautiful green. Hence the lusters of light, yellow-tending leaves are yellow-green.
Trees lit by sun and air: a blue side and a lustrous side
A dark-leaved tree lit by both sun and air shows on one side illumination from the air alone, which partakes of blue. On the other side it is lit by air and sun together, and the part the eye sees struck by the sun appears lustrous.