If you can get your hands on a copy of the chapter in Forgeworld's Imperial Armour Model Masterclass Volume One, they did a scene of a nasty, wet, muddy, artillery entrenchment. If my book wasn't in storage I could scan the chapter and email it to you. (It does look like it is in a .pdf format. Google it and you should be able to DL it.)
There was also a good tutorial in Shepard Paine's book: How to build Dioramas (Kalmbach Publishing) on how to do mud with celluclay.
Not sure if you want to use pigments(diorama might need a lot of pigments) but I found this:
http://www.secretweaponminiatures.com/index.php?main_page=page&id=5
Edit: Found DLs of the two books. The FW one uses a DIY filler to model the mud. Shep Paine recommends a papier mache (celluclay). Brush it on the base, then use the models to put imprints of footprints, treads, wheels etc... while the material is still wet. Once dry, paint it the color you want (they also mention adding pigment/color to the wet material) Then coat with a gloss varnish for a wet look once it's dry.