To follow up on this thread, there is an article about "Human skin color diversity is highest in sub-Saharan African populations".

Rather than stating that there is one true African/Black phenotype, this study shows that skin color variations are highest in "Sub-Saharan Africa".

"Published data on skin reflectance were collected for 98 male samples from eight geographic regions: sub-Saharan Africa, North Africa, Europe, West Asia, Southwest Asia, South Asia, Australasia, and the New World. Regional differences in local within-population diversity were examined using two measures of variability: the sample variance and the sample coefficient of variation. For both measures, the average level of within-population diversity is higher in sub-Saharan Africa than in other geographic regions. This difference persists even after adjusting for a correlation between within-population diversity and distance from the equator. Though affected by natural selection, skin color variation shows the same pattern of higher African diversity as found with other traits."

"http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11126724&dopt=Abstract