The prescribed color for the stole for confessions is violet.
Wearing the wrong color, or for that matter no stole at all, does not invalidate the absolution.
There can be times when a priest might wear the wrong color stole. It might be all he has available if he’s travelling. It might be that he’s already vested for Mass and someone wants a last-second confession. Of course, there could be other times as well. White can always be used if the proper color is genuinely unavailable.
Nevertheless, a priest is obligated to follow the norms of the Church when he celebrates the sacraments.
No priest has any right nor any authority to decide for himself to intentionally wear a green stole when he hears confessions.
He is obligated to wear a violet stole, and he is obligated to ensure that there is one available for him to use.