Dark- stress usually made by high traffic area's where there is too many humans commuting, eye contact with another chameleon etc.
Pale with mouth open- overheated get a lower wattage bulb, and mist more often
Bright coloring when exposed to another chameleon- Normally this is only so with the males if they are puffed up and very bright it means they are threatened by another chameleon or by you, females usually just turn dark when exposed to other chameleons unless she wants to mate, in that case she will be very bright pink (receptive) or very dark(unreceptive)