The U.S. Army used a crude numbering system for its paints that dates back to the late WW2. They have called Olive-Drab shade #9 through the 40's, 50's and 60's even though the actual paint entered the FS (Federal Stoock) number system in the 50's. There is considerable confusion on the WW2 shade because the airplane modeler and real aircraft restorers have suckered for a different shade which was introduced in WW2 but never produced in quantity before war ended. They maintain that the shade was used but reliable sourses say it never got to the troops or the aircraft. Testor's OD (34087) is closer to the WW2 shade but should only be used for U.S. Army helicopters in Vietnam. Only those used 34087 in the 60's, the vehicles being universally darker.
When the MERDC color system came out, using four colors, two of them covering major areas, and two covering less tham 10% of the surface, Olive-Drab had its number changed to #8 because one of the lower number colors was dropped and the ones with higher numbers were lowered in number. A four-color paint chart for each type of vehicle was produced for troops to follow using the new numbers. Olive-Drab was no longer a primary camouflage color in the 1970's, having been replaced by the main collor called Forrest Green. It still was used on equipment but not very often on vehicles.
A summary (brief) starts at WW2. The shade was then called #9 Olive-Drab. This was a flat color and Sherman tanks and such were painted in this color at the factory. It is very close to U.S. Army Corps shade Olive-Drab #41 and the model paint I use which comes close to that is Tamiya Olive-Drab, however you can find similar paints using USAAC #41 shade in other paint lines.
The paint authorized but never used was shade #319 Olive-Drab and that color is frequently missused by modelers fro WW2 subjects. #319 Is the color that did not get to the equipment or troops before the war ended but it is widely claimed to be the correct light Olive-Drab shade. I have never found a reference that says #319 is the shade for Army vehicles which makes sense since it was developed as an Army Air Corps requirement for a lighter Olive-Drab to be used in North Africa.
By Korea the Army was still using darker (I call it dark to keep from confusing it with 319) shade that was essentially the same WW2 vehicle color. However after WW2 the flat sheen was gone and the paint was now a semigloss shade and that made it look even darker. This paint would be the first to get FS number using the first system called TT-C-595 dated from 1950. The TT-C system attempted to consolidate all known Olive-Drabs into a single color given the numbers 2430 for semi-gloss and 1405 for gloss and 3412 for flat (lusterless in government lingo). The flat shade was almost never used.
FS595 comes from 1956 and it changed the numbering system and numbers but not the paint colors themselves. Olive-Drab #9 was still specified on the unit paint charts and tech manuals but this purchased using FS numbers which were now 14087 and 24087 and 34087. Both were dark shades and essentially similar to the WW2 shade. The first number is gloss and the second semi-gloss. There was no flat shade (34087) in the first FS595 books.
So then the Army dropped shade #7 called Loam in 1956 and all higher numbered colors dropped one number. The manuals now called Olive-Drab #8 instead of #9 but the shade did not change. Vehicles in the 50's used 24087 for this at all times except for VIP and special vehicles which used the gloss shade 14087. Nothing with wheels or tracks was to be in a flat shade. Units, particularly armor units, liked to add black to the color so even darker vehicles are common in the 50's and 60's. This was a unit decision made by the local commander. Darker vehicle paints made by mixing were most common in units stationed in Europe for some reason, but the mixing was never autorized at high command levels.
Prior to Vietnam the U.S. Government developed a need for a interim flat Olive-Drab and it published revised versions of FS595 chip books called FS595a. These included a new flat Olive-Drab which was given the number X34087 to indicate the color was new. This shade did not match the previous colors and was lighter and yellower. It would be changed to 34087 in later books (confusing it with the previous 34087 from the first book) but the color never matched two darker colors already in widespread use. Testor's paint is a match (pretty good too) to the later 34087 chip and you can buy other paints that come close. I used to use Floquil's version for some helicopters and Testor's for others since the shades wer slightly different and the helicopters were different depending on the weathering state of paint and such things as wax applied by the crewchief.
However vehicle units retained the darker shade. When the Army developed a new "camouflage" scheme for its new jet helicopter fleet in 1965 it used the new spec FS595a X34087 as the base color. Since the chip did not match the darkness of the gloss and semigloss shade the new aurcraft were lighter and more yellow looking when compared with the vehicles in the same army. Even helicopters in Vietnam showed this with those having high-viz schemes based on gloss 14087 shade appearing much darker than their camouflaged cousins with subdued markings. The dark shade died out on Army aircraft by 1968, by which time all the inventory had been repainted in the dull 34087 scheme.
Vehicles continued to be painted in semigloss Olive-Drab 24087, until MERDC four-color camouflage system started in the mid 1970's. Curiously, touch-up paint cans were available in the Vietnam years with 32087 in them and these were fine on the helicopters but did not match on the tanks.
So, I would use any shade similar to Tamiya Olive-Drab (XF-62) as a starting point and adjust it to match your subject. The dark OD's intended for USAAC aircraft in WW2 will work also so you should have choices to pick from. But keep in mind shade 319 is not correct for these things because it was not produced in time to be used in WW2 and was not used at all on Army ground equipment. 319 Was an aircraft color used by post-war Air Corps and Navy and may have continued on those into the 1950s where it was used for anti-glare panels and such.
As a good way to judge, the standard WW2 M1 helmet issued to Army troops was painted in #9 Olive-Drab. These can be seen in museums and in collections or military surplus stores still in their original paint. The color of the helmet in WW2 was the same as the vehicles in that war as well as most of the aircraft. The Korean war-era paint was nearly the same shade (as far as we can tell) exept for looking darker because of the semi-gloss sheen. Once it was dulled by the sun it looked more like the WW2 color.
Having said all this, I am not a nit-picker for correct colors as long as they are in the ballpark. I also find it strange how much people will weather this color since U.S. Army was famous (amongst it's troops) for keeping things painted with fresh paint. Only abandoned vehicles took on the faded look seen on many otherwise excellent models. Dirt would, of course, make the vehicles change color and lighten the apparent color, but underneath the dirt was still the same rich, dark Olive-Drab the Army always used.
Today, the FS595b books do have 14087, 24087, 34087 shade chips at all. The nearest matches now are FS595b shades 14084, 24084 and 34084. Army helicopters are using a special paint that is low refelctance which comes close to these shades and is much darker than the Olive-Drab used in Vietnam. It does resemble the original dark shade from WW2 but not perfectly.