There are some very colorful killifish. You can also go with dario dario but they are quite small and require special foods (but from what I've seen of your postings I'd say you're at a pretty advanced level of the hobby and can handle it lol)
There are also some quite striking gouramis to be found or you could always go African cichlid depending on the tank size. I kept mbunas in the past and they are BEAUTIFUL.
EDIT: strike the mbunas didn't notice you said 10 or 20 gallon.