Modern US vehicles are painted in (usually) 4 of 12 basic colours in what is known as US Mobility Equipment Research & Design Command (MERDC) schemes. The first 2 colours of each scheme cover about 90% of the vehicle roughly half each, with the secondary 2 colours about 5% each. Black is a secondary colour for all but the full white Artic scheme.
MERDC Colours in enamel H = Humbrol, TTM=Testors Model Master
Black FS 37038: H 33, TMM 1749
Dark Green FS 34102 (or 34082):H 117, TTM1713
Desert Sand FS 30279: H (4 parts 121+ 2 parts 118 + 1 part 110), TTM ?
Earth Brown FS30099: Neither known ?
Earth Red FS 30117: H 186, TTM 1701
Earth Yellow FS 30257 (or 33245): H (1 part 94 + 1 part 63) TTM ?
Field Drab FS 30118 (or 33105): H 142, TTM 1702
Forest Green FS34079: H 116,TTM 1710
Light Green FS 34151: H 151, TTM 1715
Olive Drab FS 34087 (or 34088): H (4 parts 108 + 3 parts 163 + 1 part 33), TTM 1711
Sand FS 30277 (or 33303): H( 6 parts 168 + 2 parts 118 + 1 part 110), TTM 1704
White FS 37875: H 34, TTM 1768
Some Humbrol colours may not be easy to find these days.
This comes from another site, apologies I can't remember which. Hope this helps.
PS If you want the low down on what schemes require what colours, drop me an email.