A big difference in time periods and Area of Operations determined the finish on USMC Helos. In VN many helos were repainted or painted Stateside with a semi gloss Green.
This paint was not the same used for Fleet Marine Helos that were more subject to the Elements of Fleet use. ( Salt water for one). Most USMC helos post VN were in fact basicly used in the Fleet and were painted accordingly with a higher resistant paint. ( Also more expensive and harder to maintain). I would hazard a guess that this would cause a shade change in the intended color. I agree with everyone posting here. there was a definite color or hue difference between these two periods.
A satisfactory paint has always been a huge problem for USN and USMC helos.. More so than Airplanes because of the nature of how Helos operate up close and dirty with Sea Water etc.