Haha, I just bought some Pennywort today, but after seeing these two links, I don't know which it is. I only knew about the marsh one, so I assumed that was what I got. I hope it's the marsh one, since I planted it, but it had loads of frilly roots everywhere just like the Brazilian one.
What about a dwarf lily - nymphaea (or nuphar) stellata? It's not all that colorful, but mine gets a lot more color when I use root tabs. I like the arrow shaped leaves. It's flower is the national flower of Sri Lanka. It's not tiny, but it's not huge either. Here's what I'm talking about: