According to “Army Aviation in Vietnam, 1961-1963” written by Ralph Young:
External color- Overall Gloss Olive Drab FS. 14087.
24" “ARMY” lettering- Gloss White FS. 17875.
8" high legend “U.S. ARMY” and the radio call sign- Gloss Yellow FS. 13538.
Interior- Gloss Gray FS. 16440.
Seat frames- Lusterless Gray FS. 35440.
Early seat webbing and cushions- olive drab.
Later seat webbing and cushions- red.
Rotor blades- Flat Black FS. 37038 on the undersides; light gray (FS. unknown) on the tops.
In theater in 1962, the “ARMY”, 50" and 30" National Star Insignias on all but the fuselage top (30") were overpainted in Flat Olive Drab FS. 34087.
The fuselage top Conspicuity stripe was originally Gloss White, but other colors were used in particular red.
There were two camouflage patterns applied to the H-21C.
The first was applied to 8th TC and 57th TC H-21C’s. On top of the two OD colors listed above, tan and light blue-green. In the 57th there was no apparent pattern from aircraft to aircraft. FS. colors are noted unknown.
The second, from late 1963 to mid 1964, was tan over OD, predominately tan.
I hope that is helpful to all H21 modelers.