Of the fish I've got, you could consider dwarf neon rainbows or black neon tetras. Their colours are more subtle in that you don't get the big splash of bright red of the neons or cardinals, but I think they are better in your tank than they look in photos or even the shop.
The rainbows look pretty ordinary and then the light hits them and you get a look at their beautiful pale blue colour and the red of the fins (for the males). There's an element of suspense involved - I've had visitors look at my fish and suddenly go "wow, look at that!"
The black neons just look like they've got a white stripe in photos, but it's really a fluorescent pale blue. They've also got a flash of red in their eyes. Their colour in real life is better than the photos depict, I think.
Also, it isn't just the colour of the fish that appeals. Both of these fish are really "friendly" and swim around the whole tank. I'm a bit disappointed with my rosy tetras, who like to swim at the back of the tank so I don't see them as much. My rainbows and black neons are always at the glass if I come by (want food, no doubt!) so I feel I "get my money's worth" with them. Even though they aren't vivid coloured, you might consider them anyway.