Todd,
Thanks for the answer, interesting point. My thoughts (and I certainly don't want to contradict Mr. Connelly!) are:
1. The 5-NG/5-OG mix he has the color chip for looks like it might be based on a mix of Navy Green and Ocean Gray. Ocean Gray would be 5-O. 5-OG is Ocean Green and is a light green color. So I think MTB Green would be a light/medium green color.
2. I wonder when that color measure was promulgated? My reading indicates that at the time of the 109's service in mid-'43, forward conditions in the Solomons were still pretty primitive and they would have used whatever forest green sort of paint they could get.
I could, of course, be COMPLETELY wrong here, but those have sort have been my thoughts. No doubt your sources have a lot more access to creditable material. You're right, of course, that it makes for an interesting discussion!
Michael