Most saltwater fish have smaller scales so their color looks smoother and even as if it is painted on. Also different diets and the fact that they display bright and varying colors for identification to others of their species. Some are just brightly colored to say I am poisonous do not eat me or to say I'm a cleaner here I am. Also to blend in with their surroundings. The coral reef is a very colorful place. On the same note though there are some very colorful freshwater fish (chiclids, flowerhorns, Umbees, etc.) There are so many different reasons.