Painting 200 exhaust tips?

Has anyone ever painted a 200's exhaust tips? I'm thinking of doing mine black. I think they're some kind of metal, what paint and process would you recommend?
 
I got my diffuser with the tips already painted but i'm sure the guy said he just scratched the surface slightly with some sand paper and made sure they were clean then put a couple coats on. They actually really suit being black and it seems to be sticking okay.