While you can lighten your skin with makeup, you should only go a few shades lighter. Otherwise your skin will look fake.
As for doing cosplay makeup on tan skin, it's no different than makeup for fair skin. There are lots of things you can do. Focus on your eyes, skin, lips, it doesn't matter. Find the techniques and shades that work with your skin, hair, cosplay outfit, and you will look fine.
If you are stumped, find some one on YouTube. Maybe you share the same skin tone, or you like the way they do their makeup. Play around and have fun. It's only makeup; if you dont like it, you can wash it off!