Traditionally, Commander decks tend to be tri-color.  I don't have many tri-color legends, though, so I favor dual-color.  It really depends on the commander.  You can only use colors that are in your commander's casting cost, so if all you have are monocolor commanders, then you can only make a monocolor deck.  I don't recommend it, though.  With the singleton limitation and the focus on longer play length, multicolor decks do seem to play better.  Still, I've toyed with the idea of making a monocolor Zombie commander deck, and I'm sure a monocolor Elf commander deck has or will be tried.  More fun, I think, would be a five-color Sliver commander deck.