Iphone12 pre-order

You have 14 days to return the phone for full refund. That's what you'll have to do if the phone is being delivered to your home. Return the phone to your Apple Store for refund and IF they have the color you want available, buy the phone in the color you want. If they don't have the phone, you'll return the phone your ordered for refund and order the color you want.

It is possible, IF they have the color you want available when you go to pick up your phone. If they don't, you can decide to not pick up the phone and order the color you want for a later date pickup.