I would suggest that you invest in a handful of flesh heads, you never know when you will need them again, and it matches the real Superhero heads. Plus some of them look pretty sweet.

If it was Harry Potter or Star Wars you could get away with it by using the older official heads, or even re-coloring the newer heads, but I think it looks bad when people change the skin tone of licensed figures that have always been flesh.

It's just a distraction when they're mixed, at least to me. So please don't do that. It's ok when it's used as a big plot point, but I don't care for when folks are just supposed to ignore it and move on.