I've had the green tint on my note 20 ultra from the start. Only in low brightness and on greyed backgrounds. I think its a well known issue as I've read a good few people with this. It's also known on the tab s7+. Mainly a oled issue. Some people have sent them back and received a new one with the same issue. Some get lucky and don't have the issue. Or they can get a new one that hasn't got it as bad as there last device had. Me I just put up with it or I have turned the brightness up to where it doesn't show. If yours is a very bad green tint you can return it if you still able to ofcourse. I've read that people have sent there tab s7+ off for repair but it was sent back with no change.
I guess I was lucky with the s7+. I haven't seen the green tint. I noticed it on the note but its not that bad its just slightly green hue and only on greyed background. One reason it didn't bother me. As if I sent it back I may have got one with the green tint more noticeable.