I am particularly happy with the mud effect on the tracks. it also covers some of the defects found on the Warlord resin casting.
I use tetrion filler (just my preference - other fillers are available). I add a small amount of sand - quite fine - and burnt umber paint, with white to suit. I give this mixture a good stir, and then gently stir in some static grass.
The static grass musn't be thoroughly stirred in - it needs to show in the final mix. I then slap it onto the painted vehicle, picturing the rotation of the tracks and wheels, and imagining where the mud would build up, or get splashed onto.
The effect of the static grass can be seen more clearly on my Sherman tank:
The culin cutter has clearly been ploughing through the bocage on its way to St Lo. I finish the mud with a brush-full of gloss varnish (once the main tank has been varnished), and this gives an appealing wet-look to it.
Hopefully that may be useful to somebody. I'll probably photograph some more of my Bolt Action troops and publish them later on in the Blog.