Your better off using actually mud.
This is quite easy,
Get some Elmers glue, Dirt, water , and some color additive. Pastels or paint?
( your choice )
mix the dirt, with water and then some ellmers glue,
mix it together, then some black. Enoguh to make it look like actually wet mud.
after that to make it look fresh, take some Water resin or something glossy, add it to the mixture.
lay it out and let it dry for a while then go back and add some more water resin to make puddles
Hope this helps you
for snow, use baking soda