well the topic title speaks everything...well eg. mud on tank tracks and wheels etc

I use the following for dry European mud:

Tamiya Light Earth - for colour.
Polyfilla (UK name for houshold crack filler) - for body and adhesive.
Static Grass - for body.
White glue (PVA) - for adhesive and to give plastic property during application.

This I apply quite liberally under sponsons, fenders around bogies etc. The mud in reality dries quite quick on AFV's and light in colour. Obviously the thicker the mud the longer it takes to dry, this can be simulated using watered down PVA glue.

Even in V cold conditions the old mud will appear dry if frozen on the vehicle. So if you are modelling a vehicle from the cold days of the Ardennes offensive, a lot of the old mud will appear dry and light in colour.

I drive a Landrover spending some time off road, and a quick look inside the fenders backs this theory up.

What I have used is mud! Some dirt , white glue and water. Mix it all up and applied with an old brush. Leave it as is for the wet look or matt finish and weather for the dried mud affect.

I use more or less the same principle. White glue, water, acrylic earth colour, baking or baby powder and some static grass. I brush it on with an old brush. On tracks I brush most of it back off again with a damp brush so as not to hide too many details. Then when totally dry, I add different colours with pastels.

I use a simple putty / brown acrylic paint mixture. Mix the putty and paint so that it's thick enough to hold form then dab it on the desired area with a thick bristled brush. Once it's dried I dry-brush a series of shades of brown starting from dark and going a little lighter with each coat. I works fairly well and it's a quick process. Hope this helps!

00ooooooo..thank guys

Here is a good article about mud

http://www.ipmsstockholm.org/magazine/2001/05/stuff_eng_tech_dirt.htm