Yeah the way I do the mud is very easy you dust the part first then put some pigment fixer on wet put your light mud pigment on by tapping the brush then do the same with the darker mud pigment then your good to go.
Actually the best way to start is to find a car heavy machinery transport trailer or truck and take photos. Google should supply photos if you can’t get out so that you can decide what look you want. With tanks it will be heavy, metal tracks and weight being the major factor. But you also have general wear and tear so plenty of bare metal that’s bluing and rusting. So it all becomes bare metal, blued metal (German grey is good for this), and rusting metal, dull not bright. The trick is less than more and graphite powder is really helpful in giving the metallic feel, but not turning everything “bling”. Start with photos.