Ok i decided to elaborate on my technique here. It works best on vehicles and boots.
What you will need
Pure sand (cant help with the origin, my dad had a bucket of it in the garage, should be a finer grain, not a bigger grain.)
Acryllic paint (your choice, white for snow, brown or brown/black combo for mud)
Get a good portion of paint in your bowl. After this ligthly pour a little sand in. You will see it kinda soaks up the color. Pour the sand in until the sand doesnt soak up the color. Once this is achieved, mix. The consistancy should be a little thick, but watery, good enough that it will glop, lighlty brush on.
Apply the mixture with brush/ q-tip lightly to the sides of the vehicles tires, the fenders, ect... Just try one spot at first. Once you see the results, you can decide on more or less.
For boots, well i havent tried it, but i suggest a little more watery of a mix.
While painting on the mud, stuff will drip off the brush. The process is messy, so do it some where liek a garage or out side.
Once you have achived the desired results, dull coat the whole mudded area. i used about 2-3 coats, the stuff is pretty rock hard.
I have used the snow technique on a diorama (sideways log) it worked well.
I used the following brands of paint/dull cote. Use what you like, as long as its acryllic, just incase. The last 2 are craft paints, they work pretty good, avaible at a hobby lobby or a michaels probally, not sure, its my moms stuff (was).
Testors Model Master Dull Cote
Anita's all purpose Acryllic Craft paint (black)
Americana Acryllic paint (light cinniamon)
I use the 2 previous paints, as the black was 8 fluid ounces, for only $2, the little thing is somthign liek 89 cents. Might not be Jo Sonya, but it gets the job done.
Hope this helps alot of you in your weathering.
Pictures are in the replies below (examples)
Here is the rest of the pictures of the finished jeep. (i fixed them after Cuda's post)