Well, if you want mud packed in the tracks of a tank I like to do this. Thin whatever earth tone you want to use, with some thinner. (Simple Testers model paint is fine.) and add it in to some baking soda/powder. Its important to remember to NOT thin it down to much or when it dries it want dry hard, but brittle.
The hardest part is application. For me, mud splatters are the hardest effects to render. Some times it works just to “fling” it on to your model. But the "arching" mud pattering you see on wheele wells of vehicles from mud "overspray" is nerly imposable for me to render.
I wouldn’t necessarily put any grass in it. For one reason only though. I Have worked extensively around tracked heavy equipment and light armored APC’s (M-113 to be exact.) I have never seen grassy mud get packed in the tracks. I’ve seen it get stuck up in the wheel wells but that’s about it. And the tracks dont spen fast enough and are well enough inclosed in the track housing to NOT through it. Now if you where building a 4x4 of some sort then that woudl be difrent.
BUT you never said what affect you where looking for in the mud. (packed or flung)
Also. If its for a diorama, try plaster aperies (sp). Make it thick so you can mold it like putty, but not so thick its not milky smooth. Build up your road or field or what ever scene you want and place the tank in the mud pressing down firmly. Lift it out, move it down a length and place it in again. You should affectively be making a “trough” that your tank has driven through, with the “mud” swishing out from the tracks to form ruts