I dont know if they had a lot in stock. They were going to just "replace" my black. I asked about the white and whether or not it was painted on. They guy said, no, its just straight aluminum. I asked if I could just return this phone and buy the white. He said had to see if they had white in stock. He checked and said they could do that. So he passed me off to a sales rep and told her the situation. She returned my black and i bought the white.
Go to the apple store first. That will be the fastest if they have it in stock. They might keep some in stock for the sole purpase of exchange and returns. I know they were out of stock of earpods, but later i had to take mine in because the remote wasnt working, They did have extra earpods for replacements for that very purpose.
If you call apple they will just email you a return label. You mail the phone in and wait for your replacement (same color). In order to switch colors you have to return your phone and repurchase (within return policy time frame)