If I want to trade a black iPhone 5 to a white one on sprint how would that work?

I just want to trade, I ended up liking the white one more so I want to know much it would cost

They won't do it. And Apple won't either.

If you have had the phone less than 30 days, you can trade it. It's $650 if you have had it longer. There's no functionality difference in the colors. You might want to consider a white case.