Based on the e-mail from the order confirmation for in-store pick up below:
Can I change my iPhone order when I pick it up?
Depending on availability, you can make some changes, such as color or storage size. But you can’t replace an iPhone on a wireless plan with an unlocked phone. If you’d like to cancel your order, we can issue you a refund at the Apple Store.
If you paid in full price, and they have a color of your choice, they can do an exchange.