I agree with Mikaila31 above. Rainbowfish simply aren't nicely coloured when young. In addition, you need approximately equal numbers of boys and girls to get the males (which tend to be brighter coloured) to their maximum coloration.
As with any other fish, there's some degree of environmental context to consider too. Bright substrates (e.g., pale sand or God forbid painted gravel) will make these sorts of fish fade their colours. Shady (i.e., well planted) tanks with dark substrates encourage best colouration. Diet is important, and the more crustacean and algae foods you get into them, the better their colours will be. So-called colour enhancing foods work to encourage red colours, but don't do anything about the other colours. So you still need to balance the diet with other foods, e.g., algae flake, live daphnia, etc. to get the best colours.
Cheers, Neale