Hair color is the pigmentation of hair follicles due to two types of melanin: eumelanin and pheomelanin. Generally, if more melanin is present, the color of the hair is darker; if less melanin is present, the hair is lighter. The tone of the hair is dependent on the ratio of black or brown eumelanin to yellow or red pheomelanin. Levels of melanin can vary over time causing a person's hair color to change, and it is possible to have hair follicles of more than one color on the same person.
The full genetic basis of hair color is complex and not fully understood. Regulatory DNA is believed to be closely involved in pigmentation in humans in general, and a 2011 study identified 13 DNA variations across 11 different genes that could be used to predict hair color.
Two types of pigment give hair its color, black-brown eumelanin and reddish-brown/reddish-yellow pheomelanin, synthesized by melanocytes.