When I was a child we used old wool clothing to make patchwork quilts, big squares sewn together and backed by dark flannel. One way of cleaning them was to place them on new snow after a storm. Leave for a while. When you picked them up, the dirt was absorbed into the snow. I guess you could call it "freeze dry "cleaning. You could always have hem dry-cleaned when needed. Fading on to the surrounding colors could be a problem.