I haven't done my research but I'm sure it's melanin. I'm fairly light in terms of shade yet my upper lip , the part that remains, is pretty dark. But I have seen brown and black people with pinker lips. My upper lip from what I can remember was consistently the same color, as was my lower. I'm thinking the multiple shaded lips is more common in those with darker skin.
As for Dwayne, I could see how people think that but I'm sure he doesn't have it.
And yeah, I never had and hope for lip surgery to cure but I was wondering if it would visually provide relief. I don't think I've seen any before and after pics of people who went through with the procedures.