Human hair color is the pigmentation of human hair follicles and shafts due to two types of melanin: eumelanin and pheomelanin. Generally, the more melanin present, the darker the hair. Its tone depends on the ratio of black or brown eumelanin to yellow or red pheomelanin. Melanin levels can vary over time, causing a person's hair color to change, and one person can have hair follicles of more than one color. Some hair colors are associated with some ethnic groups because of the observed higher frequency of particular hair colors within their geographical region, e.g. straight, dark hair amongst East Asians, Southeast Asians, Polynesians, some Central Asians, and Native Americans; a large variety of dark, fair, curly, straight, wavy or bushy amongst Europeans, West Asians, some Central Asians, and North Africans; and curly, dark, and uniquely helical hair amongst Sub Saharan Africans. Bright red hair is found in some European populations, and hair turns gray, white, or "silver" with age.