A recolour just won't cut it, I'm afraid. The problem with trying to add racial diversity to the cast is that when you start getting your token black guy and token mexican, people often begin looking for stereotypes, and start pointing things out even when that wasn't the intention. If you just recolour a white trainer black then that will evoke blackfacing more than anything, which would probably offend more people than it would please.
If you want an ethnically African trainer, you'll have to sprite him from scratch.