The colours in hair, skin and eyes, are caused by the presence of melanin. Melanin is deposited in hair shafts, in the form of microscopic granules which can vary in shape, size and arrangement, which gives a variety of colours.
There are two chemically different kinds of melanin: eumelanin and phaeomelanin. Eumelanin granules are thought to be spherical in shape, and absorb almost all light, giving black pigmentation. Phaeomelanin granules, are thought to be elongated in shape, and reflect light, in the red/orange-/yellow range.
Several genes can cause variation in the density of the the melanin granules, so other colours can be produced. The most variation is found in the black-based (eumelanistic) colours.
The following table lists the commonly accepted names for the basic colours, by genotype:
Mutations of the gene for Black give rise to Chocolate and Cinnamon. These colours are thought to be due to a smaller number of eumelanin granules in the hair shaft. These are alleles at the (B) locus; Chocolate is recessive to Black, and Cinnamon is recessive to Chocolate.
A mutation of the gene for Dense colouration produces Blue, Lilac, and Fawn. These colours are due to clustering of the particles of pigment in the hair shaft. This is called dilution. Blue is the dilute form of Black; Lilac is the dilute form of Chocolate, Fawn is the dilute form of Cinnamon.
Dilution is a mutation at the (D) locus - dilution is recessive to dense colouration.