Question about coloration in tropical species.

[trilinearmipmap]

Here is something I have always wondered about but never had answered for me.

Why are tropical fish and other animals so much more brightly colored than temperate species? (On average, of course there are a few exceptions).

It seems to me that dull colors that help camoflauge an animal would help both in avoiding predators, and sneaking up on your prey. The only reasons I can see for an animal to be brilliantly colored would be to advertise that it is highly poisonous, or to attract a mate, or to mimic another highly venomous creature.

Anyway if someone has an answer to this question I'd appreciate it.