Colour change

When did you purchase the phone? If more than 14 days ago, you can't return it and Apple is not currently accepting iPhone 11 in trade. If you purchased your phone less than 1 days ago, you can return the phone for full refund and buy whichever phone you want.

You can certainly sell the phone privately and use the money you get for the phone towards the phone you want.

If your iPhone is less than 14 days old, you may exchange it. Otherwise, you would have to sell it and purchase a new iPhone.