Melanocyte Stimulating Hormone, or MSH, is the hormone responsible for controlling melanin production in your skin.
MSH is produced by an intermediate lobe of the pituitary gland. The darkening of skin is a result of granules of melanin which spread through the branches of specialized melanocytes called melanophores. The granules are carried outward along microtubules using kinesin as the motor. They assemble at the actin-rich periphery of the cell carried by myosin. The granules are carried back to the center of the cell along microtubules using dynein as the motor.
(1) Webpage: Melanocyte-Stimulating Hormone (MSH). (http://www.mobiusweb.com/~jkimball/BiologyPages/M/MSH.html)