SR, yeeeup, use real mud. Take your regular garden soil (preferably dry), add a wee bit of water, then add a generous amount of Elmer's white glue, mix mix mix, then spread it like so on the base and you have real mud! When it dries, it'll look like sh1t. Really, I'm not kidding. It'll look like dried doggy poo. That's when you start giving it a nice dark wash to make it look wet. Wet mud is different from dry mud in the color, not the glossiness of the wet mud. Wet mud is darker than dried mud. You either paint the mud with a dark color, or use your used thinner that you used to clean your paint brushes and airbrush then just brush it on the surface - if you're feeling adventurous, pour the thinner! It'll make the mud dark and it'll stay that way. And look! Wet mud! :-)