Gray hair is typically more difficult to dye because it is courser and dryer. As a follicle ages, it produces less sebum, which is what gives hair its moisture and shine. Coffee, because it stimulates follicles and improves hair health, may be better able to coat stubborn grays. If you are attempting to cover grays, it’s best to leave the coffee on hair for twice as long to give it enough time to moisturize and penetrate the course strands. Remember that coffee is not a permanent dye for any hair.