When I was restoring my WWII jeep I did some research on the the proper OD paint. What I came up with was that there was two different shades a dark and light. When they changed I don't know. What I do know is that Model Masters is the lighter shade.
What's the right color? I don't think anybody really knows. Like Robert said weather takes its toll along with the fact that these vehicles were repainted often in the field and the soldiers didn't take alot of care as to how or what they used to thin the paint with. Along with different manufactures making the paint, and there you have it 47 different shades of OD.
I say if you like the way it looks, It right.