A dye is a material used to impart color to a substrate such as fabric or hair. Natural dyes have been used for this purpose for thousands of years. Egyptian clothing dating from 3000 b.c. was dyed with indigo, yellow, red, and green colorants. These dyes were derived from from plant and occasionally animal sources. Dyes from plants included blue dye from the indigo plant; blue dye from woad; red and brown dyes from the madder plant; yellow, orange, brown, and black dyes from various trees; orange or red dye from henna; and yellow from safflower and weld. Animal dyes included red dyes from the cochineal insect and Tyrian purple from shellfish. Dye baths were made by mixing the dye with an alkali, perhaps wood ash, and stale urine, a process noteworthy for the odors it produced. (Despite the smell, urine was a convenient source of...