You'll have to see on the morning of your pickup if there are any available in the color you want. If they have available stock, they will let you change. If they don't and that's certainly a high probability since millions of people pre-ordered the 12 Pro Max, you can return it for refund and re-order the phone you want.
This is from personal experience, btw. When iPhone was released, my partner ordered the blue model for pickup in our local Apple Store. Just before brining out the box to him, he asked if he could change to the green color. They didn't have any available as everything in stock was already pre-order fulfillment phones, so he chose to keep the color he ordered as the only other option would have been to return the unopened phone for refund and order the green phone.