It's pretty simple. All the paints are Vallejo ModelColor except for the shading wash, which is Citadel.
I start with a base coat of Dark Yellow, and add ragged patches and spots of Flat Brown and then German Camouflage Dark Green, to about 70% coverage. The green patches kind of curl around and partially envelope the brown.
Then a coat of Citadel Agrax Earthshade wash to provide some shading, which also slightly darkens the underlying colours. One overall coat is probably enough; if more darkness is required in some folds, the second wash must be applied more precisely as pin-washes or else it will darken the base colours too much.
Last, I add small spots of Dark Yellow in clusters on the dark patches. The spots shouldn't be distributed too evenly, but in bunches of 3-5 with some separation between them. I found I had to do this step after the wash, or else the spots tended to fade into the background.