Brian,
For shadow effects on the field grey a mixture of 830 Field Grey and 862 Black grey or you can use 979 dark green.
Painting faces can be trial and error, the best way I have found is to experiment untill you have the desired effect but basically these are the colors I use.
The base color I use is 804 Beige Red,
for the initial shadows, a mixture of 40% 941 Burnt Umber and 60% 804Beige Red,
light shadows ie around cheeks and cheekbones, etc, a mixture of 804 Beige Red, 981 Orange brown and Cavalry brown, about 1/3 each.
for medium shadows mixture of 60% 941 Burnt Umber and 40% 804Beige Red,
for dark shadows 941 Burnt Umber
Highlights 804 Beige Red and 955 Flat Flesh
High Highlights 955 Flat Flesh
It pays to experiment with these color to obtain the effects you are after. I usually work in 120mm scale, for 1/35 scale the effects could be slightly different. One word of advise do not use straight white for the eyes, tone it down a bit with blue or yellow.
Hope this helps.