Skin
From Memory Alpha, the free Star Trek reference.
Skin is the external covering of a lifeform, consisting of a membranous tissue made up of skin cells. Skin is the largest body organ found on humanoids. The study of skin, in terms of its structure, function and skin-related diseases is known as dermatology.
Historically on Earth skin tones were often used to justify many heinous acts and prejudice, such as the problems of racism encountered by Benny Russell in the 1950s. (DS9: "Far Beyond the Stars")
Skin color is often a distinguishable trait between different alien species and has been the focal point of slang terms such as pink skin or "monotone" to more extreme xenophobic actions such as war, one such skin color driven war destroyed the population of Cheron. (TOS: "Let That Be Your Last Battlefield")
The Borg Queen grafted skin onto Data's android frame in an attempt to make him more susceptible to the Borg Collective's seduction. (Star Trek: First Contact)
