When they have the color swap, they're called "shiny" Pokemon. Incredibly rare, it's impossible to find one in the wild. [While Vanitas is talking, his Zoroark has stepped back enough to be out of sight and transformed into his trainer, making... well. Stupid faces. Pushing his cheeks together to make fish lips, all that.]

I'd say anything reflecting his personality is the way to start. You can always change their name later if neither you or him care for it much.