My way to do this is to brush on a mixture of plaster/water/brown acrylic paint/wood glue onto the wheels, tracks, fenders,... I use various old brushes to acheive different effects.. The reason I add paint into the mix is to reducethe amount of painting required afterwards... Plaster being white or off-white, it's not always easy to paint it 'mud' colour once it has dried on the tank. With my way of doing things, a few coats of washes are all I need to get a nice looking mud on my AFVs.
By the way, I use trhe same mixture on my base... Looks better if the tank and the base share the same mud!!!!