There was a pretty exhaustive article by William Archer a few years ago in FSM. The early war shade of OD was Olive Drab 41, and is a very dark brownish green. Tamiya's looks close to me, but Archer says it is too green. About 1942 or 43 a new standard was adopted, which was close to the Army Ground Forces standard of the time, and was a greyish, lighter color than the OD 41. The Testor's Model Master enamel and Acryl, as well as the Polly Scale and Humbrol 66, are close to this shade. Neutral Gray was also replaced at this time by a lighter gray, near to light Gull Gray or Vietnam era undersurface gray. The old British magazine, Quarter Scale Modeler, had an article about this also, and said that the FS equivalent for OD 41 is not 34086 or 34087, but 34085, which is browner than the other two. Armor used in the Korean war was the chocolate brown referred to by Wayne Baker.