The standard USAF flightsuit from the vietnam era was sage green. It was a green with a substantial grayish cast to it. As it weathered, the tint became more grayish.
RAF Interior green, slightly darkened with a medium gray would not be a bad for a well-weathered Nam -era flightsuit. Modelmaster 34096 (B-52 green) with a little light gray wouls give you a less weathered flightsuit.
The colors varied considerably depending on age, exposure to the sun, what they were laundered in (i.e. clean water or a bucket of sludge from a paddy or ditch), and even dye lot variances. The keyword here is to mis medium greens and med-light grays until you get what looks right to you, then go for it. If you're putting more than one crewmember on displat with the F-4, make sure there are subtle differences in the color/shade of each flightsuit.
Good luck with the project.
"The world would be a much simpler place if every one could pick and choose their obligations, but we can't and we shouldn't." Major Charles W. Whittlesey