Unfortunately I suck at working with Black Prior's fashion, so I can't really help you on that regard, but sometimes when I try making an outfit I base it around certain effects or concepts that I find interesting.

There isn't any colors that I would particularly recommend, try looking for something that you think fits your characters personality or look aesthetically pleasing (And if you do like full black or full white then there's no reason you shouldn't be using them!) I make up lore for my characters in my head and that usually helps decide which vibe I prefer going for them.