Black walnut hulls contain a pigment called juglone, which is chemically similar to the pigment found in henna leaf. This chemical protects the tree from disease and competing plants, and lends its astringent, antimicrobial and anti-fungal qualities to various skin care preparations and cosmetics, such as natural hair dye and self-tanning tanning lotions. Black walnut hulls are also used to produce a natural brown dye for ink, wool and other textiles.