weathering

Tried something new and it worked fine. I did a desert scene and wanted the sand/dirt scaled about right. I put some thought into it and decided to try some flour. I spread some carpenters wood glue on the ground surface and poured the flour on, patted it down , dumped the excess and had wonderful results. A little sand paint and I had a winner.

There are a variety of methods:
For mud, I usually use the real thing. I sift very fine dirt from my back yard, and make mud out of it using water and white glue, then just splash on the model, based on reference pics.
As for dust, there are a few things you might try:
As mentioned above, either pigment powders, such as those from Mig, MMP, or Quartermaster will work fine. A similar (read: same) method is to buy pastel chalks, grind them up into powder, then use that.
My favourite method is using tamiya Buff paint. I can't remember the exact XF number.
Take the paint, and thin it. Thin it a LOT. I'm talking about 1 part paint to 6-7 parts water, if not more. If you have an airbrush, take this tint, and lightly 'mist' it over the model, concentrating on the lower hull and tracks.
If you dont have an airbrush, I suggest thinning the paint even more, something like 1 part paint to 10 parts water. Use this to 'wash' the lower parts of the hull and tracks and wheels. Do this generously. As the water evaporates, you'll be left with a fine dusty surface. You'll have hard edges from where you applied the wash, but if you're careful and attentive, as the paint is just about to go dry, you can drybrush with a completely dry brush , and attempt to blend the edge away.
Hope this helps!

I use a very thined out mixture of ft.tan or ft brown as a wash.You can brush paint or spray.With brush you my have to use a little more thinner to brush it out to the way you like.With a spray I thin to a near clear wash and spray,this gives it a good affect that is hard to beat.The paints that I use are enamals,Digger

I find that ground up pastels work very well.,

paint or MMP powders applied in a thinner will sufice. but recently on oneof my builds, i used a combo of various things. i used some MMP powders(ask around like on greatmodels or WWIImodelmaker.com) in an acrylic medium, along with dirt, static grass, sand, some foliage debris, and some celluclay with very tiny pebbles to simulate mud. i used some darker tans or browns, depending on whether it was to simulate dried or frrresh mud, and some grass to simulate other organic materials. take some lawn debris of the right size and add that intot he mix as well. make sure to get matte acrylic medium, not the gloss. the mixture can be painted after application, but its not necessary. if you want, paint a lighter color on the to-be muddy areas to simulate dried mud, and then apply the mixture over top to simulate fresher mud.

hope it helps!!

Not my expertise, but I believe putty is used to simulate mud. Flat earth or buff paint can be used to simulate dirt and dust. Sorry, not much hlep here.