Thats a stunning piece of modelling, really well done.
I really like useing pigments mixed in water. Depending on your colour, this works great for dust and mud. If i want a fresh mud look, i use the same mix of celluclay that i use on the base.
But it is tricky getting a realistic look without obscuring a nice paint job. But given the slow speed of these, i imagine the lower portions and running gea4r would be caked in it and not so much higher up.