I have read pretty much the same thing in other sources.
As Mr. Harlem has stated they kept fresh paint on the vechiles, but didn't tell you that they were painted in the field, in all kinds of weather, most likely with brushes by a soldier being punished, who doesn't have a clue about proper mixing ratios, with paint that was thinned with what ever happened to be handy
What I'm trying to say is humity, thickness of paint, how the paint is thinned and what its thinned with, all can effect the color of the paint.
In a nut shell he's right, but Model Master's olive drab (FS34087) is so easy to find and so easy to use, same color every time.