Just my 2 cents:
Unless you are modeling a specific vehicle at a specific time and have very, VERY good color photos of that vehicle, just about any shade of OD is fine on a U.S. vehicle. The exact shade of OD will vary on every vehicle over time...sometimes a very short period of time. Sunfading, reaction to saltwater environments, and how often or with what the vehicle is washed, will all cause the specific shade of the original factory OD paint job to change. A combination of these factors can have dramatic effect on the color of U.S. vehicles.
I spent 5 years stationed in Turkey where we had blazing summer sun, saltwater air, and freezing winter nights. You would be surprise how fast Humvees and AGE equipment went from a pristine new paint job to every shade of faded, oxidized, and streaked color you can imagine. During the middle of the summer, it wouldn't even take a month before you barely tell the difference between one-month old paint jobs and one-year old jobs.
Again, just my 2 cents,
Robert