I would, and do, use a dark wash, black or brown depending on conditions. When applied over the figure, the wash will naturally fall in the folds and build up more. On the high points, it will just run away to find a low point so the deep folds get more wash remaining in them than shallow ones and high points are not affected at all, instant shading.
The wash has to be vey thin and very runny but can be made up from Pigments and water, acrylic paint and water or thinners and enamels using Turps or white spirit. If you have time, you can use oil paint and thinners to great effect but there is a penalty of long drying times. The figures below were painted their respective colours and then washed (including the faces) with water based washes to bring out the detail.
It really is a quick, simple way of painting figures that are additional to a scene. If it was a lone figure, the main focus of a scene, I would put a bit more care into it.
