These are just natural colourations, like how mercury is gray.  They're not made of mercury I don't think, but that's the best example I could think of OKAY?
Base Colour:	dark tealy bluey variety of gunmetal gray
Dead:		flat gray, lighter than Base colour

These are involuntary reactions.  They can be emulated voluntarily, but it's hard to supress them when they actually get triggered.
Fear:		azure streaks going from the rear eyes and over the shoulders onto the sides of the back
Danger:		yellow and black and red with diamond shapes, some of which may be connected with vertical lines
Irritation: 	yellow with darker yellow splotches
Sexual Arousal:	red emnating from the eyes, reddest at the skin closest to them.  Usually only happens during the small window of the peak of the sexual receptivity cycle.