1/72 scale mud

putty is cheap and reliable.

You can also use acrylic gel (ship model guys use it to create water) they have the clear stuff and also gel that's mixed with a fine sand. Paint to taste, yum.
G.L.

You can also use Celluclay. Mix it with a dollop of white glue and several drops of an acrylic earth tone, then apply to the selected areas. Works great
Gip Winecoff

There's probably umpteen ways to make mud Bernie.

Here's just a couple

Use real mud and brush it, splatter it, or cram it into the running gear.
That's the easy one and it does have surprising results.

Mix modelling putty, white glue, earth tone colour paint, bits and pieces of find sand, and just about anything else you want crammed in there. Apply to the lower portion of the vehicle and running gear.
Dry brush to highlight when complete.

bernndye:
there was an article in FSM awhile back about adding mud to armor ( that was a good one, Ron Poniatowski !). i'll try to remember the issue it was in, but u may want to try a browse-by-subject in the meantime.(or ask Ron P, he's been posting lately) also check the earlier posts on this forum, I know djmodels posted re: this and there were a couple of other replies also...
best of luck gettin' down and dirty !
frosty