Catherine,
My best guess for this is Acropora tumida although it is hard to get an accurate ID from the photo. There are so many varieties and so much reticulate evolution that occurs from one area to the next it is very hard to say what the exact species is, however to deal more directly with your question, although this species isn't necessarily known for it's twin coloured polyps there are a lot of cool colour morphs that can happen in many corals, especially sps and it appears you have an oddity/rarity with the bicolour polyps. Sweet piece, I think it's a keeper.
That said, it may also be that the polyps are just slowly turning colour and may eventually become more uniform. How big is the coral and how long have you had it? And was it always like that?