I'm not an armour modeller but I can tell you one thing about all U.S.Olive Drab paints. They were remarkably inconsistent! They may have matched the relevant standard at the factory but the formulae varied and the paints weathered and faded in a bewildering variety of ways. Look at a decent period colour photo of a line up of vehicles or aircraft and this is usually evident.
A brief history of olive drab:
Before the war the US Army Air Corps used an olive drab paint designated Olive Drab No9. This was actually related to Royal Flying Corps Green,a WWI colour used by the British.
In 1940 the US Army and Navy got together and introduced a new ANA standard. They decided that O.D.No9 was too light and introduced a new paint designated Dark Olive Drab 41. Just to confuse things this "new" colour had been around since 1932!
Now you'd think that was it but no,the original O.D.41 was a mixture of seven different pigments which was fine in peace time. When WWII came along it didn't need Einstein to discover that an awful lot of Olive Drab paint was going to be needed. Manufacturers set about simplifying their formulae. As a result different manufacturers used different formulae,some used as few as two pigments. The problem was the different paints weathered very differently.Pilots of the 14th Fighter Group operating Lightnings in North Africa reported that under African sun their early-model P-38s turned,rather disconcertingly,into a bright purple colour! This obviously wouldn't do.
So,in March 1943 yet another standard was set, ANA 613. This standardised Olive Drab to the Army standard which had developed,despite being officially the same,into a lighter shade than the OD 41 used by the USAAF. Now you say everyone will be using the same paint. I'm afraid not. The Army did indeed use the new standard ANA 613 (which was the same as their interpretation of OD 41 anyway) but the Air Force showed no interest in it and most aircraft manufacturers continued matching to the earlier,darker,OD 41.
The long and short of this is that there is no need to beat yourself up about an exact shade of a particular Olive Drab. There never was one!
There is a good,armour orientated article on the mysteries of Olive Drab here.
http://www.militarymodelling.com/news/article.asp?a=4536
Regarding your original question on colour call outs in kit instructions,they vary a lot. Many are well researched,like the
decals,and will give a manufacturers colour (or mix ratio) which will certainly be close. Some are absolute rubbish.
Cheers
Steve