I'm not going to go throught the effort, but I'm sure you'd find that all the other rectangles are colored according to the proportions laid out by L dot N. That is, .9659, .866, and .7071 from the brightest rectangle, which is the one perfectly facing the light.
 

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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)