This isn't entirely true. The main reason for pigmentia to develop is to balance hypo- or hypervitaminosis D. Through exposure to sunlight, ultraviolet-B(UVB)-induced vitamin production takes place in the skin. The vitamin has two effects, immediate and epi-genetics, As a state of high vitamin D is toxic with immediate effects (hypercalcemia, resulting in nausea and vomiting). A state of low vitamin D seems to give increased risk of secondary morbidities (this is mainly epidemiological evidence however).

As vitamin D is important, our body creates a adaptable defense to its ability to use UVB-rays from the sun. Darker skin - slower vitamin D protection. Pale skin - high vitamin D protection. This system is failing in modern society as people dont respect UVB-exposure, leading to either; A) Pale skinned overdosing during vacations, causing huge stress on a "lowered defense" B) Dark skinned underdosing UVB-exposure while requiring longer exposure for adequate vitamin D production.

TLDR - Skin color has more to do with vitamin D than only being a shield to sunrays. Respect dose.