Swapping iphone colour

You have 14 days from date of purchase from Apple, to return a device for refund and then purchase what you want. But Apple no longer sells iPhone 11 as it was discontinued last September, so you are likely way past that time frame. If you bought the phone from someone other than Apple, you'll need to see what their return policy is. Likely not any longer than Apple's return policy.

Depending on where you got your phone and when you got it, you might be able to return it for a refund and then buy the phone you want. Just note that Apple does not sell the iPhone 11 model anymore.