I've found that since you don't want to get paint on the snow (that would ruin the effect, even on the edges) you should do the base in layers, just like would actually happen on real terrain.
I put down earth, a few rocks, paint them black, then dry brush them as I want them to appear before I even add snow or mud.
For both, I make a paste. For mud, I use a basing earth mixture (A combination of different soils for realistic texture. Yard dirt, sand, potting soil and dried coffee grounds
atm.) pva glue, and gloss brown craft paint. (The gloss makes it retain a wet mud look.)
For snow I use pva glue, fine white or light tan craft sand, and white paint.
Eon of Battle's channel is a great resourse for learning how to base;
https://www.youtube.com/user/EonsOfBattle/videos