For their game color line, I used Yellow Olive for a base followed by Cayman Green, and (I think) Camouflage Green for the highlight. Various mixes in between each color were needed to make the transitions less jarring...it's not quite a P3 Triad, in other words.
I haven't done an olive-drab with their Model Colors yet, but several of the greens suggested previously would work. Soviet Armor would be one I'd take a look at as well.