Hair coloring products can be divided into a number of categories, conventionally characterized by the length of time the color will remain on hair. There is an extremely wide variety of human hair colors, varying from the palest Scandinavian blonds to the dark blue-black of the Far East. The color of human hair is due to the presence of melanin pigments. In hair, the pigments are located only in the cortex. In broad terms it appears that the blond, gray, brown, and black shades are due to eumelanin, and the red and yellow shades are due to the pheomelanin pigments. Eumelanin is the generic term for a variety of dark-colored natural pigments. These pigments are generally intractable solids which are insoluble in all solvent systems. Pheomelanin pigments are largely responsible for the red and yellow colors of hair and feathers.