I don't know what the green player is in warthunder
As @Jcraft153 was saying in the comments above. The green players are in your Squad that you are (as of the Patch 1 year'ish ago) automatically assigned to at the beginning of each match. This is all according to Good Ol' Reddit. It can be toggled on and off in the settings. BUT it is a good feature to keep on because you receive a "Teamwork" bonus if in close proximity of your Squad mates during a firefight. I hope this helped! Happy Hunting!