Once the iPhone goes multi-carrier, they could do a special color to differentiate between carriers, ie red for Verizon, orange for AT&T (with black still available for all carriers).
Better still; Apple should sell there new phones exclusively through the Apple Store and mail order as unlocked phones and let AT&T, Verizon, and other cell phone carriers compete for customers with service contracts that supplement handset costs. This would be a win for Apple and for customers, and this kind of healthy competition between carriers would ensure faster and continued evolution of cell phone service throughout the world.