Hair can be naturally darkened with strong, black coffee. According to Whole Magazine, darkening hair with coffee is significantly less damaging than using chemical-based hair dyes.
The easiest way to use coffee to darken hair is to brew a strong pot of coffee and let it cool down completely. Completely saturate hair with the coffee brew. Let it sit for a few minutes before washing out. Repeat the process until desired color is achieved. Wash hair with regular shampoo and conditioner after dying it with coffee. Maintain naturally darkened hair by repeating the process once a week. Black tea and black walnuts are also ideal hair darkening alternatives to chemical dyes.
Coffee is used to darken hair as a temporary experiment, but a long-term color change is unlikely. Coffee is not acidic enough to penetrate the hair cuticle and coat the hair with long-lasting color, nor does coffee have the chemical properties that would enable it to penetrate the cortex.
For dye to color hair permanently, it must penetrate both the cuticle and cortex of the hair, strip away the existing pigment and replace the pigment with a new color. If the dye penetrates only the cuticle, then the color is considered semi-permanent; the color fades in about six to eight shampoos.