What is your water like?
If hard water choose shellies.Not the most colorful(L.ocellatus"gold" is about the most colorful) but the coolest little fish you can find. I love mine. A small colony or a trio would do well in a 20g.
If soft water, choose dwarf cichlids. They are very colorful. Rams,Kribs, Apisto. caucatuoids(sp?) are a few of the more common ones that would work well.
What are the dimensions of a 20g?