How to change Contact Card background color?

You can't make a global change. You can only change contacts one at a time:

1. Open the Contacts app (or the Phone app and go to Contacts).
2. Tap the specific contact you want to change.
3. Tap Edit in the top-right corner.
4. Tap Contact Photo & Poster (or tap Add Photo and select Monogram or a photo/Memoji).
5. Tap Customize, then select Poster.
6. Tap the color circle icon at the bottom of the screen to pick a new background color.
7. Tap Done and confirm your changes.

if you’re talking about your card then go to settings and click on your account and then click edit if you’re talking about your contacts click on the phone app and then click on contacts then click on the contact you want to do and then click the edit button then click on the icon the contact has then you pick what you want to add