Hi Dick, I usually just use a mix of suitable earth colours (with white if they are supposed to be dried mud) so it depends on which paints you have. If the mud is wet you should make the colour darker and also give it a subtle sheen so that it looks like it's wet!
If you don't have any dark browns you can mix one from scratch using red, yellow, black and white.
Just in case you want thick splatters you can simply mix in a small amount of fine sand/soil and even a little plaster to the paint, for a three-dimensional effect.
Einion