I've had this argument many a time on MacRumors, and some people have claimed I'm wrong (I'm yet to see any proof of this), but changing the colour of a phone at the genius bar will cause significant inventory issues, and this won't be done. It is possible to do it (they *can* do anything). It's like how they *could* just give you a MacBook if they wanted to. There's nothing stopping them. But they wont.
The *only* way to change the colour of a phone is with a retail swap, for a phone bought from an Apple store, with the original receipt within the first 30 days. That time limit will only ever be extended under VERY exceptional circumstances, and you would have still have to have bought it from an Apple store.