Dried mud color...

You can try this http://www.missing-lynx.com/rare_world/rw05.htm 

Mig says to mix in plaster with some pigment powder, but this is not necessary, unless you want massive and thick coverage, like for a base!

If you want dry mud, then mix in some Matt Varnish with your chosen pigment colour. If you want wet lookin' mud then you've guessed it, mix in some Gloss Varnish. Both these mixes can be mixed to any consistency you like, by addin' more or less pigment!

Maybe, to this mix, you could add some very fine grain sand or very small and fine, dried pieces of grass from your garden, for a very realistic consistency to your finish! Always remember, less is more! Hope this helps!

It depends on what you want to do.  If you want caked on mud, you might want to lay down some plaster or squadron putty on the muddy parts of it.  But if you want like a light coat of mud, you can do this

This is a DML T-34 I did a little while ago.  I dry brushed the layered light mud with tan polyscale paint.  I tried to make it where it was as clean as possible because it doesn't have any caked on mud, just thin dry layed stuff.  Mud and dirt is darker when it is wet and lighter when it is dry.