Highlights of Leaves: Why Yellow Foliage Gleams Green
Trees lit by sun and air; the bluish lustre of dark, polished leaves like the elm
Continuing the discussion of leaf colour, this page explains that leaves tending to yellow cannot produce a blue lustre, because the blue of the reflected sky combined with the leaf's yellow yields a beautiful green, so their highlights read as yellow-green. Leaves of dark colour lit by both air and sun show a bluish illuminated side and a lustrous sun-struck side. Because leaves commonly have polished surfaces, they act partly as mirrors of the air, and dark leaves such as the elm's, when not dusty, give highlights partaking of blue.
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Yellow leaves gleam green because blue and yellow mix
Leaves tending to yellow cannot make a lustre partaking of blue, since everything seen in a mirror partakes of the mirror's colour. The blue of the air reflected in a yellow leaf appears green, because blue and yellow mixed make a most beautiful green; therefore the highlights of bright yellowish leaves are yellow-green.
Dark polished leaves like the elm's show bluish highlights
Plant leaves commonly have polished surfaces and so mirror the colour of the air, which itself partakes of white through thin transparent clouds. When such leaves are dark by nature, like those of the elm, and are not dusty, they return highlights partaking of blue.
Trees lit by sun and air: blue side and lustrous side
In dark-leaved trees lit by both sun and air, one part is lit by the air and its illumination partakes of blue; the other part is lit by air and sun, and the part the eye sees struck by the sun appears lustrous.