It seems whenever there's an odd couple or lovely angels with different hair colors, the blonde will be tanner while the brunette is of paler complexion. Generally, this is due to the stereotypes associated with the hair colors. In fiction, blondes are more sociable/lively and trendy, thus they're tan from either always being out somewhere or from tanning. Similarly, the brainy brunette is believe to be more studious and less fashion conscious (aka a "stay-at-home" person), giving them less reason to be tanned; either that or they're goth and actively tying for a light pallor. In Japanese culture, the blonde-tan and pale-brunette contrast has an even stronger cultural cannotation; raven hair and pale skin are seen a sign of traditional beauty (pale brunettes are especially common amongst yamato nadeshiko, thus a tanned woman (especially one with light hair) is thought to be either an athlete (swimmer usually comes to mind) or (more likely) a materialistic gal/ganguro. Or course, the characters need not have the stereotypical personalities to fit this trope, sometimes the producer or artist just liked the visual contrast. In animation, it could simply be that a tanned brunette might be mistaken for ambiguously brown rather then just a white (or Japanese) person with a tan and a pale-skinned blonde might look rather generic due to the hair of gold trope being so old and well-known. See Also: Dark Skinned Blonde, Black And White Beauty and Eerie Pale Skinned Brunette.
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