Say a water damaged or mossy look on a wall surface is desired. Paint the area in these consecutive colors: black, dark green, dark tan, medium green, then your primary wall color, tan. Let the piece dry fully between coats. Scribe panel lines with the back of a broken xacto knife blade and a straight edge. Then lightly sand on a small area where the stained spot is to be. Do this in a downward motion to recreate a seeping of water effect. Use what ever colors look natural for the type of wear you are trying to achieve.

To save paint on large projects, map out where you want your stains to be and just layer the paint in those spots.