Depending on your Carrier you have a window of time they allow for returns. Some are 14 days and some are 30 days. I don't know about exchanges. But you can return it and then buy the white one if they won't exchange it. If they'll exchange it within the window of time then that's great.
You should check with your carrier for this information. You will need your receipt and I believe if it's within the window of time for your carrier you should have no problem returning it and getting the one you want provided supplies are available.
Also, after you return it you can buy the one you want directly from Apple.