I use Testor's Model Master Radome Tan for the basic skin color...
I use burnt sienna acrylic washes for dark shadows, and Radome Tan, lightened with white, for the drybrushing of highlights, and oils for the lips..
I don't paint the whites..Ever.. At the average viewing distance, the whites are the same color as the skin-color on real people, so don't do it....
For the iris, I paint a narrow stripe of color, either blue or brown, right through the center of the eyeball, letting go past the eyelids top & bottom.. Doesn't matter, cuz I'm gonna paint this area anyway..
Using a sharpened toothpick, I add a black dot in the center.. Then trim the eye-color back to the top and pottom..
Remember that eye are almond-shaped, not round or football shaped... If you paint them as arches top & bottom, it'll give your guys a "pop-eyed" look that ruins the eye.. Also, only the bottom 2/3rds of the eyeball should be visible..
Another thing to remember is that soldiers are outdoors, in the sun, and have a tendency to be squinting.. Goes doulbe for groundcrews on the ramp at the airfield.. Use this to your advantage...
Remember too that soldiers in field also wear camouflage paint on their faces quite often... Use THAT to your advantage as well.. Light Green and Loam are the colors for Woodland areas, so call it dark green and light green.. Or even black is added too.. Had one guy that used to do his face-cam every morning as Gene Simmons of KISS, lol... Light Green overall and Dark Green "batwings", lol..
Actually, it looked kinda cool under a Kevlar...