In areas where the sun is usually obscured by clouds, people are said to be lightly pigmented to allow more efficient Vitamin D synthesis. This, if true, is nevertheless only part of the story. A sunshade isn't going to cause that much social upheaval. We are uptight about skin color because it is a major organ of communication stretching back hundreds of millions of years through our ancestry. Before examining human skin color, it would be best to take a brief look at the importance of coloration in other species.
Some fish communicate with the skin.
They contract minuscule color cells that dramatically create, destroy and recreate other patterns. One second the fish is bright red in threat, the next second it flees and blanches completely silver-white. Reptiles, especially lizards, also use the skin for communication by varying their color pigments. Dominant males distend their red throat wattles, raise their skin hackles, and turn black. The subordinate color is a pale, washed out green.