I've been pondering this. Logically, I would think that the rear window panels would be painted whatever color the aircraft was originally ordered in.
If there were any field modifications to the factory paint scheme then that area would remain the original factory color.
I'm referring to B and C models, of course.
So far in the pictures I've seen of pre-war P-40's the area is O.D.
I assume that the AVG aircraft, being ordered from the factory in Brittish camouflage, would likely be painted in camouflage behind the glass panels as well. Since the camo patterns were not field applied.
So I suppose it is up to us as the artists to research the subject, as per the norm, and determine how we choose to paint the subject.
I think it would safe to assume that not too many of these planes were left with the pit color behind the rear panels. But that's just my opinion based on my own understanding of the article provided.
I also like the camo being behind the rear panel and not pit color so I don't think I'll be painting that area pit color anymore.