Art store pastels are a cheap way to get pigment powder - make sure they’re wax-free and then crush them up! Apply where needed with a brush in very small amounts. Spray fixative or hairspray can hold them but it’s not really necessary. Be aware that varnishing or spraying them usually changes the matte finish which defeats the purpose - washes and inks may be better for glossy brown dirt. You can also directly rub them along edges of scenery pieces or models for a very quick effect.