Collect brown walnut hulls from your black walnut tree in gallon buckets. If the walnut hulls are still green, wait at least a week for them to change color.
Cover the hulls with cold water and let them steep for three days. For a darker color, transfer the hulls and water into pots and boil them for one or two hours.
Strain the liquid through cheesecloth or nylon stockings into glass storage containers. If not using the dye immediately, add a dash of white vinegar to the liquid or freeze the containers to prevent mold growth.
Paint or spray the liquid onto wood to stain it dark brown. If dyeing fabric, steep fabric in the liquid dye overnight.