In my case I went/going with the following for the 3 Color Cam Pattern:
503-322 "Field/Pea Green" = Tempo 34094 "383" Green
503-321 "Flat Olive Drab" = Gillespie 34087 "Khaki" Olive Drab
512-301 "Flat Black" = Tremclad Flat Black (in Spray Bombs)
I also used Gillespie 24087 Semi-Gloss Olive Drab for my Engine Compartment etc, as this was the original base coat that the M38A1 and M151A2 were delivered with. My M38A1 CDN2 was still Semi-Gloss OD in the Interior and the Engine Compartment, with the 3 Color Cam Pattern painted over the Semi-Gloss OD on the Exterior surfaces.
All 3 of the M151A2s that i've had were base coated over the Semi-Gloss Olive Drab with either the 503-322 Field Green or 503-321 Flat OD inside and out, and then had the Black and the other Green/OD added to make the Cam Pattern. Only the Engine Compartments were still Semi-Gloss OD. One was sprayed with Red Primer over the Semi-Gloss OD prior to the Cam being added. The other two were brushed right over the Semi-Gloss OD... (horrible).
Paint is a major source of frustration when dealing with the 3 Color Cam Pattern. It gets beat around on here quite often. In my case, I gave up looking for the holy grail of perfection and went with the 90% solution. I have collected literally hundreds of color photos of the pattern, have a parking lot full of vehicles at work that still have it, as well as have a can of each of the original paints. The problem is as stated in other posts was the manufacturers subtle differences, condition of the paint itself (old, frozen at some point etc), how it was applied (brushed, rolled, sprayed, cut with gasoline, thinner, varsol) and how the vehicle was stored.
There was clearly two (slightly different) versions of the 503-322 Field Green floating around at some point. It seems out east it was more "Pea Green" than out west where it was a bit darker Green, closer to the Flat OD. In my case, the 383 Green was darned close to the "East Pea Green" 503-322 Field Green, so I went with it. Honestly, in the right light, you'd be splitting hairs to tell the difference. Again, that is the Tempo version of 383. With Gillespie, Sherwin Williams or PPG it could very well turn out different.
EDIT: Picture added. I like this one as its the only photo I have that really captures the "true color" of the 383 Green. Thee rest of the pics I have don't look right with artificial light. If you look really hard at the scrap MUTT on the right, there is some original 503-322 Field Green showing on the Inner-Fender above the rear Wheel. Its a long shot, but believe me when I say its damn close:
One of these days when its slow at work, i'm going to take a few recently procured vehicles (LUVW, MSVS, Milcot, LAV III and a Refurbished Bison) and park them all side by side and take a photo. Although they are all "CARC Green", none of them are exactly the same. If we can't get it right now, I sleep well knowing that it was imperfect back then as well
Thats my Nickle on Paint