MIG Europe Earth enamel/pigment set to help with this task. To make the mud, I combined some plaster, pigments, enamel earth effects, and some dirt from the flowerbed in front of my house and applied it heavily along the mud chutes anywhere else I thought mud would accumulate. After it dried, I mixed a similar slurry, albeit slightly lighter, and applied it sparing around the model to add a little bit of depth to the mud. I'm not completely happy with it, but I think it will do for the time being. I'll probably add some more straight pigment attached with binder in various places as well.
The only thing left to do touch up and weather the tracks and add the final touches to the model. I plan to have it completed before my IMPS club meeting on March 1st!