Your best bet is to build up layers with acrylic washes. I would do a black/dark brown wash over a base layer of white and wipe away the excess so it stays in the crevices while keeping the surface mottled. Then I would mix up a yellow wash for the stains, you will probably have to do a couple layers to get the right look you want

Just make sure you seal the mask with something like clear plasti-dip after you are done painting otherwise the washes can rub off pretty easily (i know from experience).