Hi Mark,
I'm also working on a WW1 bust at the moment. For the khaki I first mixed some Lemon Yellow with a spot of black to get an opaque olive green then added raw sienna until it looked about right (to my dodgy eyes anyway). My target was somewhere between green and brown, maybe leaning a bit more towards brown. I used Vallejo English Uniform as an undercoat which was a good guide for the colour. I highlighted with base mix + Naples Yellow Hue and shaded with base mix + Raw Umber.
My project crept into a recent post about a drying box so I'll post a link in next post so you can check it out. Its not a great photo but it should give you an idea and you can decide for yourself if you think it looks right or not.