Some fish such as these Creole Wrasse stand on their heads as a signal to the cleaner fish (a juvenile bluehead in this case) that they want to be cleaned of itching parasites (Bonaire).

 

These are goatfish in their normal color. When they want to be cleaned they go to a cleaning station and turn red as a signal that they want to be treated (Bonaire).

 

A juvenile Emperor Fish. This youngster is not recognized by its parents as belonging to the same species because of its totally different color, and is therefore allowed to live in the same territory by the adults which would chase away any other Emperor Fish from their territory because they cannot allow competition for food. The young ones are not competitors as they have a different diet from the adults. By the time the fish is large enough to conquer a territory of its own, it changes color to the adult phase, as well as its diet (Indo-Pacific).