Hi there.
I use both Mig pigments, and dry pastel powder. I get good results by applying Tamiya X-20 (enamel) thinner to the area I'm about to muck-up, then apply the pigment (with the same brush). I let that dry and knock off any excess with a big soft makeup brush. After that hit it with a pin wash of a slightly different color to add depth, and once sealed, it's good to go.
Another method is to apply a pin wash to the areas (which is mostly thinner, right?) and then dab on the pigment, dry, and seal.
Lastly, if you want more texture in the mud, add some plaster into your pigment, and mix enough thinner into it to make the consistency you want. If you apply some glossy paint over it in areas after its dry, you can replicate wet mud pretty effectively.