Why do my fishs' colors vibrancy change so dramaticly?

i got a festivum cichlid that changes colors quite dramatically when responding to certain stimuli like food, another fish intruding on territory, or when something moves very close to it unexpectedley. it goes from light to dark brown with yello stripes in only a few seconds. its very cool.
 
I have a small angel fish who's stripes disappear when he's stressed or when he's in a place in the tank where there are no plants. I just noticed this the other day. I also had neon tetras when I was younger that turned almost completely grey at night. I think it's just a natural reaction to environment and stress.
 
AquariaCentral.com