Took off the leaf shader (now it's just a square) and just made it simple Diffuse with a magenta (red and blue) color. I made the pawn a yellow (red and green) color. The two colors only have red in common. If we see red, it's bounced between both objects.

This is what I got with shadows off for the square - this looks right

This is what I got with shadows ON for the square - this is completely bogus

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)