I have no objection to your original crop; and looking at the pic a second time now, after a few hours, I'm inclined to agree with you about leaving it alone.
To me, the green on the seagull's underside is more likely to be an indicator that something green directly underneath was reflecting photons upward. But even given the light really had been green, we'd simply be doing Mother Nature a favour by neutralizing it. ;)
Good work tracking the gull, BTW. I know how quickly they can cross one's field of view while seeming to be doing nothing more than a leisurely glide.