I also doubt there's going to be a significant difference in the prices. There's enough of every color available; it's not like there's a "Limited Edition" color.
Its not when he buys the iPhone, but what color for a better resale. No one knows so why bother with the question in the first place. No one knows. Just buy the darn thing. I read the gazelle site, and it showed that it was the Carrier that made the difference in price one gets, not the color.