Another piece of science:
The earliest members of the hominid lineage probably had a mostly unpigmented or lightly pigmented integument covered with dark black hair, similar to that of the modern chimpanzee. The evolution of a naked, darkly pigmented integument occurred early in the evolution of the genus Homo. A dark epidermis protected sweat glands from UV-induced injury, thus insuring the integrity of somatic thermoregulation. Of greater significance to individual reproductive success was that highly melanized skin protected against UV-induced photolysis of folate (Branda & Eaton, 1978, Science201, 625–626; Jablonski, 1992, Proc. Australas. Soc. Hum. Biol.5, 455–462, 1999, Med. Hypotheses52, 581–582), a metabolite essential for normal development of the embryonic neural tube (Bower & Stanley, 1989, The Medical Journal of Australia150, 613–619; Medical Research Council Vitamin Research Group, 1991, The Lancet338, 31–37) and spermatogenesis (Cosentino et al., 1990, Proc. Natn. Acad. Sci. U.S.A.87, 1431–1435; Mathur et al., 1977, Fertility Sterility28, 1356–1360).
As hominids migrated outside of the tropics, varying degrees of depigmentation evolved in order to permit UVB-induced synthesis of previtamin D3. The lighter color of female skin may be required to permit synthesis of the relatively higher amounts of vitamin D3necessary during pregnancy and lactation.
Skin coloration in humans is adaptive and labile. Skin pigmentation levels have changed more than once in human evolution. Because of this, skin coloration is of no value in determining phylogenetic relationships among modern human groups.
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WJS-45F4X4V-13&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_version=1&_urlVersion=0&_userid=10&md5=c70b4804251d1e749613802a0f9eb94b
And another:
The primate face and rump undergo colour modulations (such as blushing or blanching on the human face, or socio-sexual signalling on the chimpanzee rump), some which may be selected for signalling and some which may be an inevitable consequence of underlying physiological modulations. Because for highly social animals like most primates, one of the most important kinds of object to be competent at perceiving and discriminating is other members of one's own species, we investigated the hypothesis that primate colour vision has been selected for discriminating the spectral modulations on the skin of conspecifics, these modulations providing useful information about the current state or mood of another conspecific.
.....
This connection between bare skin and colour vision may be important in understanding why humans are the ‘naked ape’: for primates with colour vision, skin modulations may serve as signalling on any body part that can be seen (e.g. a chimpanzee rump), and for apes that walk upright, more parts of the body are potentially visible and amenable to colour signalling. (See §3 of the electronic supplementary material for further discussion of face bareness and also see §4 and figures 2 and 3, for a discussion of evidence of the visibility of skin colour modulations.)
http://http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1618887
So skin colour:
Presence of increased levels of melanin offering protection from UV exposure
An indicator of diet and potential vitamin availability/defficiency
Closely related to evolution of primate colour vision enabling spieces recognition and mood prediction (useful for instance in distinguishing anger from sexual receptivity).