I'm almost positive this has probably been suggested before, but in case, I thought I'd bring it up (if not again).
Anywho, I think there should be some way for creatu to be bred between each other. it would have to include at least 2 of the same creatu (colors can be dif), and PERHAPS a slight increase in chance for rare colors to pop up? And perhaps it could take a few days for the breeding process to successfully work, and even then have a certain % chance that the process failed? I don't know, just an idea of how to keep people from breeding a lot and getting more colors.
Like I said before, it's most likely been proposed a few times before, but maybe they didn't offer the limitations I suggested along with it? Not sure, anyway, tell me if you support or not :)