It varies from cham to cham, so as you get to know him better, you will be better able to tell his moods. Generally, I would say that dark or intense stripes indicate some sort of stimulation - could be fear or nervousness, could also be excitement, but it's a good sign to give your cham some space if his colors pop suddenly.
My jax would put on his stripes when he was scared or angry (to the point where he felt he needed to defend himself), but up to that point, his head would turn an intense blue instead of showing stripes. It was really very pretty, but that was his sign that he was upset. Until I really got to know his moods and what made him uncomfortable, it was difficult to know exactly what he was trying to tell me. I just tried to give him space if he suddenly brightened up or changed body language (tensing, puffing his throat, etc).
Some chams will pop their colors in the sun, too, or when sleeping (putting on their PJs), and some only show very subtle changes - they never throw the bright "danger" colors. So that's why I say it's not an "across the board" translation.
