Once again thank you for your comments guys.
Mike the face camouflage is a combination of pastel chalk and oil paint mixed with white spirits. I literally mix the oil paint and pastel chalk into a paste and then apply it in globs to the face. I then use a brush filled with white spirits to wash most of the pastel chalk and oil paint away. Hopefully what's left behind is a stain and a smudge that is not too dark and thick. I let the face dry for 24 hours and then reapply the paste. Hopefully after 2 or 3 days it starts to create the layered effect I was trying to replicate.
It's a process I've used on a number of occasions with varying success. On this face, I've used both black pastel chalk and dark green, mixed with black oil paint. I know from my own experience of being in the field on operations that you don't need to apply camouflage crème to have a face that looks like this. Just being hot, sweaty, dirty and tired all the time will make you look like this.
Last night I painted a series of dilute washes of clear Satin Gloss over the face to create a slight sheen to the skin. I will then add some very small spots of Gloss Varnish to replicate beads of sweat to complete the effect.
Wayne, the resin base supplied with the bust is perfect for the job. It's painted and glued to the bust now, so there's no chance of swapping it for a wooden base.