So I just skimmed through Live Aquaria and found some fish that live up to my color standards.....
Dwarf Gourami
Flame Dwarf Gourami
Red Tailed Shark
American Flagfish
Peacock Gudgeon
Scarlet Gem Badis
Boesemani Rainbow
Don't anyone freak out on me! That's just a list of fish I found colorful, not ones I researched and planned on putting together. Shoaling fish aren't really anything I'm interested in, unless a fish I just love needs to be in groups, like the rainbows. I guess I wouldn't mind of shoal of rummynoses, if I had the room.
Dwarf Gourami and Flame Dwarf Gourami are the same fish, different colour morphs. Only the males are particularly colourful, you
might get away with 2 males in a 55g, and then again.....
Red Tailed Black Shark tend to get very aggressive as they get older. I'm not an expert on them but I think even a 55G may be a bit on the small side for them. SOmeone else help me out here.
American Flagfish. Just plain don't know. Soz.
Peacock Gudgeon are a beautiful fish, looked wonderful in my 15g. Just upgraded to a 25g, and they're starting to look a little lost, kinda "Oooh, where's the peacocks?" I would think a pair, or even a quartet would be lost totally in a 55g.
Scarlet Badis are TINY, like less than an inch, so if the peacocks look lost, gawd knows how these would look. Again, only the males are colourful, and the males will squabble with each other when kept as a group. However, in a large tank, they would have space to make their own territories, so a group of males may work.
Endlers, again, tiny, you may well not ever notice them, but they are peaceful as anything.
Bosemans would suit your tank superbly, I believe they are a shoaling species, so would need at least 6, but a group of 8-10 would look stupendous.