Couldn't there be a check box when the game begins where the player chooses which skin tone to use? I understand that you don't want to put a dark head on a light body or vice versa, so you need to have all of the people be the same shade, but with a check box at the beginning of the game, you'd simply use all dark people or all light people.
That still wouldn't give you diversity within a single play of the game, but at least people who are dark-skinned in real life could play a family or tribe that looks like them. It would double the size of the graphics files, but it shouldn't make a lot more work for the artist, since simply darkening the existing files would take very little time or effort.
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Middle-aged professional woman still plays computer games. Film at 11.