a school of blue tangs and a school of yellow tangs is nice but needs a decent sized tank...
I like big angels, but they need big tanks too
there are some small species of groupers that only get 3 inches long and they are respectivly, bright blue, bright yellow, bright red. very nice peaceful easy to keep fish, just hard to get ahold of.
in a marine tank its pretty hard to not have a couple fish make a diverse colour combination...