Hair color changes result not only from alterations of melanin production but also from changes in the hair structure itself, altering its optical properties. A variety of genetic, metabolic, nutritional, and acquired disorders result in hair color changes. When the underlying defect can be corrected, hair color usually returns to normal. The flag sign can occur as a result of nutritional insults or due to medications. Most drug-induced changes in hair color result in lighter hair color, although PABA and some chemotherapy regimens have darkened hair. Green hair due to exogenous copper may be associated with prior damage to the hair cuticle. Alopecia areata may selectively involve pigmented hairs. Regrowing white hairs have shown both keratinocyte and melanocyte abnormalities. Gray hair may temporarily darken after inflammatory processes, after electron-beam-induced alopecia, and after some chemotherapy regimens. Much remains to be learned about the physiology of human graying.