Make the Dye
Put on protective rubber or vinyl gloves before you begin handling the walnut hulls. Any substance that can dye your hair can do the same to skin or clothing, so you may also want to don an apron, plastic poncho or other covering to keep the rest of your body safe as well.
Place the walnut hulls in the mortar and pestle and grind them. If your mortar and pestle is small, you may need to do this in batches. If your mortar and pestle is made of stone or another porous material, transfer the ground-up hulls to a metal or glass mixing bowl (do not use plastic, as it will absorb the color and scent of the dye, and may stain permanently). Cover the hulls with boiling water and allow them to steep, just like a tea, for a minimum of several hours, or up to three days.
After boiling and steeping, strain the liquid through a piece of cheesecloth or extremely fine mesh sieve two or three times to remove any walnut pieces. Return the infusion to a pot and boil until it is reduced to about a quarter of its original volume.
Add a teaspoon of ground cloves or allspice to the above infusion. Steep the mixture in the fridge in an airtight container for about one week, taking it out often to shake and redistribute the contents. When you are ready to use it, strain the mixture once more through a piece of cheesecloth to remove any remaining debris.
Color Your Hair
It is best to begin with clean, wet hair, so start by washing and conditioning your hair before starting to dye it. Then, while wearing protective gloves on your hands, use the above preparation to color your hair.
Pour the black walnut hull infusion through your freshly-cleaned and damp hair a minimum of 15 times. If you have enough liquid left and would like a deeper, stronger color, pour the dye through your hair several more times.
Wait 10 minutes, then rinse hair thoroughly to remove all traces of the dye liquid. Dry and style as desired.
A homemade hair dye made from walnut hulls is different than many other natural dyes because it is extremely potent. While most natural products require repeated use over time to be effective, a walnut dye is powerful enough to penetrate the hair shaft in one use, though you can use it several days in a row to achieve a stronger, darker color.