>>5015391
One observation I've made is that the color change does not appear to have anything to do with mood, at least as far as I can tell. I've observed direct reactions to light, however.
In one case, I noticed that the portion of his skin which were obscured by shadow reacted by exhibiting a lighter pigment. As a result, he was split down the middle between light and darker shades, or 'fired up' vs. 'fired down'.
So, it's possible the color change could have just been a reaction to light.