Did you know that fish first evolved in FW? The move to brackish and then SW was a later developement.
I dont agree with that statement and would like to see where it came from! IF you believe in evolution which i may and may not im not sure, well that cant be true! Just for the fact is evolution believes that the world at one time was a mass of water after the melt of the ice age....and you dont see FW
fish in SW
I believe and this is my opinion on top of what i wrote in my first reply is that I believe its for survival. Someone stated that "survival of the fittest" but he said it was for mating. Survival is refered to LIVING. We all know that most fish dont breed overly often with a couple exceptions. Lets all look at the Powder Blue as an example one of the most colorful and beautiful fish. Theirs a few major colors white, black, blue, and yellow.
These colors i believe are in certain spots for camoflage, blue is mostly the body blend with the water more easily, yellow corals and black night. Fish also tend to lighten in color during the night, possible for the fact to blend in more. Lawnmower blennies for example can vibrate their colors (have seen mine do it) not drastically but a little. An octopus can change colors to blend into its surrondings and the list keeps going on.