Lightly airbrush plain old matt black getting thicker towards the exhaust (or lighter the further away you go)?
That's what I did on my Jolly Green. Turned out OK actually, looks pretty good. Although that helo's exhausts come out the side at the top of the fuselage, not the back end like a Huey. I may or may not have posted pics at that stage (can't recall if I did), so you might see them in my build thread.
I'd also say it depends on scale, if you're doing a large scale helo I'd expect you'd want to use several colours and mix them up and have varying patches/layers. On a smaller scale helo (for example a 1/72 Jolly Green) you probably wouldn't do that as it might not look right.