Another point that may help, although Olive Drab had a specific formula, it did vary a bit with each manufacturer (Massey-Harris for example used a very light OD during WWII), also, if applied in the field by a camouflage unit it would vary even more. Add to this weathering, spilled fuel, fading, grease, dust, etc, etc, and the color would be quite different from vehicle to vehicle.
Ron