Green tint on iPhone 12 iOS 15

I have not come across that in testing,

First try a Forced restart

https://support.apple.com/guide/iphone/force-restart-iphone-iph8903c3ee6/ios

If no better

You should arrange a service or repair

https://support.apple.com/iphone/repair/service

Anything other than Screen or Battery may be a replacement phone and the price is in the article.

If not covered by warranty and you don't have AppleCare+, you may have to pay full replacement cost for severe damage 

You can see additional service information within the following article: 

Get your iPhone, iPad, or iPod touch ready for service - Apple Support