Many different ways...for permanent mud, regular modeling putty can be dobbed on areas that usually collect mud...tops of arms, bumper brackets and shocks. Use rubber or a color that is close to the area you want the mud to be. Smaller areas you want to stand out, can just use plain plastic glue, let dry and paint it brown. Oil spills from jerry cans, glue and gloss paint. Dry brush areas of traffic with dirt; entrances to hatches, fenders etc. mix in rust. on grab bars, handles, tow cables & open hatch rims. Regular dirt & mud is OK until the first time the wife catches you running it in the house.[:@]