How do you go about painting the face cam paint?
I always paint the skin first, and quite often by the time I get around to the 'men with green faces' part, totally chicken out of adding green paint; as by that stage I quite like the way the faces turned out.
It can be a tad daunting to effectively cover up something you're happy with. That said...
I've tried a few different colours:
If just brown I tend to go with 'German cam. black brown' stripes across the face. Small'ish brush & very thin paint, diagonal lines, little bit at a time as oppossed to one continuous line.
For the green option I've used several different shades (trial & error), current favorite is a broad line of very thin 'German cam. dark green', probably need a couple of coats, try to avoid going too close to the eyes in case it runs it the recess. Then fill in the rest with a very, very thin layer of 'Luftwaffe cam. green', effectively a filter over the skin shades.

For a brighter green try something like 'Uniform greem'

Hope this is some help.