Mottling or mottled skin refers specifically to vascular changes in the skin which cause a patchy appearance.
Melanin is a pigment produced in the skin cells that causes skin coloration.
An amino acid, tyrosine, is converted to melanin through a series of complex chemical steps in the skin cells. This process may be affected by heredity, heat, trauma, solar or ionizing radiation, heavy metals, and other factors. Pigment production and distribution in the body is regulated in part by hormones. The changes may be temporary or permanent. A person's degree of skin pigmentation determines, to some extent, the various dermatological diseases to which a person may be susceptible.