Depending on how accurate you want to be on a specific aircraft, FS34087/Testors 1711 is a good generic match, but the actual colors varied with any number of conditions, including the age of the airplane, and the "as received" OD paint color has historically been widely variable.
Additionally, some crewchiefs wiped their aircraft down with rags dampened in hydraulic fluid or turbine oil, darkening the color and making the finish more of a satin than matte finish. The bottom line is that the individual colors varied widely, as you can see by the pics below... HTH... Good luck!
UH-1B MedEvac, circa 66-67:
Guns a Go Go, 228 ASHB ACH-47A, circa 1965-66
UH-1B, 2/20th ARA, circa 1965
O-1D Bird Dog, 1st Cav Airmobile, 1967:
O-1 Bird Dogs, Dec 1967
UH-1D, 1967