Mine is mostly black, there is a small section of white, not sure if I wore it off (doubtful as it's in a case 99% of the time) or came that way. I doubt the sticker being white is from use, remember these are fairly new phones (Nov 2013), so if the sticker was that worn from use, you'd most likely see other evidence of heavy use on the phone.

I would discuss the issue with Google, they can provide a mfg date or will exchange it if you feel uncomfortable about the sticker.