Mud on an AFV or mud on the ground?
Here are a couple of thoughts on mud
1. Use Household Wall spackle - either mix in paint first then apply with a dental pick type tool or apply then let dry and paint.
2. Mix white glue with sand, static grass, coffee grounds (seal the dio when done), smidge of water
3. White glue and baking powder
4. Pastel dust (lighter coats)
This is one of my early attempts and the color is very differentiated, but you get an idea..

For ground work I go with household spackle on a bed of celluclay. Wet your finger and then smear it around to get the soupy mud affects.