One of the behaviours that I really enjoy seeing is how the fish use colour changes to express their state of mind.
I've noticed that one of the fish that does this dramatically is the unicorn tang (N. unicornis). This is his dark phase, which is only displayed when near the cleaner shrimp. He also has a light phase, also used when near the cleaner shrimp and a camouflage colour when sleeping. That's four very different colour schemes and he can alter them within about 2 seconds.
Look at the change in the yellow surrounding his eye:
Normally, he looks like this:
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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank:
http://www.canreef.com/vbulletin/showthread.php?t=28436