Being new to painting US tanks, I set out on a quest to find the Holy Grail, Olive Drab 9 in a bottle. I did search on various forums to see what people were using for OD, but they usually cite Steve Zaloga's Tamiya OD choice. Well, I'm an enamel kind of guy!
My two primary references were Steve Zaloga, in Military Modelling,
2002, #3, & Robert Archer's article on USAAF colors which appeared in Modeling the Second World War (Fine Scale Modeler).
In these two articles, I gathered the following: USAAF OD41 Was a shade darker than AGF OD9 (OD9=OD319=OD22, QM 3-1). Mr. Archer stated that Tamiya OD was an near exact match for OD41, & Mr. Zaloga liked it for OD9, lightened with a little ochre. So far, so good, as OD41 is supposed to be darker than OD9.
Never having seen before a real color chip of OD9, I was intrigued by this Color. The best I could do to get an appreciation for OD9 was to take Tamiya OD (XF-62) & lighten it as directed with dark yellow (XF-60). I did this in various concentrations to get a feel for this beast. To my delight, I found out that lightened Tamiya closely resembled an enamel! Testors Green Drab (1787).
As a double check, I lightened the Testors with Humbrol Ochre (83), and as I hoped, this matched Steve's second choice, Polly Scale OD (505370). Steve said that this Polly Scale was like a lightened shade of OD, & would serve well for Scaling the color down to suit your model.
But for you acrylic guys, you might like to know this: Polly Scale actually has a better choice than the one Steve tested in his article. Try USAAF OD (505224) If you like a slightly darker version of
505370. I believe had Steve tested this one, it would have been a closer 2nd choice than the Polly Scale he chose.
So, in summary, from darkest to lighter, it's
Tamiya > MM Green Drab > PS 505224 > PS505370
These colors when lighted with Ochre, are all on the same linear color line.
I hope that someone who has an actual WW2 color chip can test my theory that OD9 in a bottle just might be Testors 1787.
Lastly, the color chip in Mr. Zaloga's Stuart model book (Osprey) is very wrong. The printing was screwed up.