Hmmm...I don't now if you would count these, since they are not their natural skin tones, but since they are really popular series, i thought I should suggest these.
There is a running joke in K-ON! about how easily/dark Azusa Nakano tans. This cosplay could easily be done with a medium skin tone, preferably wearing her pink bathing suit.
There is a running joke in Azumanga Daioh! that is exactly the same, with the character of Kagura.
Currently Sewing: Black Mokona Modoki(Humanoid and Full-Body Versions) for Ohayocon 2012
Currently Planning: Miku Hatsune(Regular Outfit) for Colossalcon 2012