Hi Kemo,
Generally speaking, olive green would be the appropriate base colour, however there will always be exceptions and without more specific information its hard to give you an absolutely correct answer; for example what airborne unit is he with (82nd, 101st, Marine Recon, Delta?, what pattern uniform is he wearing?, what year is he in Vietnam?
If you could show us a pic of the model box art that would help narrow down your options, because there could be many different types of uniforms worn by US airborne troops in Vietnam.