To add dirt usea lambert with a noise texture added, then plug the noise texture into the the transparancy of the lambert (maybe with a ramp too to control where the dirt is) then use a layerd shader with the "dirt" lambert over the material you want.

You could also usea fractual for the bump mapping for mud.

Oh Choose the colour of the dirt you want via the lambet!!

I've doen this a couple of times and it looks quite good after a bit of tweeking!