This is how I make rubble. I have made rubber molds of brick wall sections, field stone wall sections, stucco wall, and cobblestones from Plastruck masters, or you can make your own. I cast plaster of paris into these molds to make thin sheets. I break these into pieces to form the larger fragments of the rubble and paint them before placing them on the diorama. I then use the smaller pieces to make the piles of loose rubble. Simply use a plastic cup, fill part way with plaster bits and pour in some diluted paint of a suitable color. Stir the mix until completely covered with paint and pour out on a piece of window screen to dry. Repeat this process with several different colors; tans, browns, grays, etc. This will give you several different colors of partially pre-painted rubble. For very large piles of rubble, place blobs of plaster, styrofoam, etc where you want the pile, then glue in place the large rubble pieces that match the color and material type of your building. Mix several colors of your pre-painted rubble together with some fine sand and sprinkle around the larger pieces. Spray the rubble piles with a water/white glue mix, tinted with dark brown/black wash, to secure it to the base. Dry brush with suitable colors when dry. The advantage to this method is that it produces a very multi-colored rubble without having to paint each and every tiny fragment a different color. I always gets a lot of compliments on my rubble at the shows, it really makes a difference in the overall appearance of your diorama.