This is pretty neat. Skin tones are pretty common it seems.
Kudos for not including #ffffff
Not sure I would call it a study, but it is interesting.
Interesting, the word 'Popular' suggests that these colours were explicitly chosen by the user creating the image, rather than them merely being present in a photograph.
I would say they were 'common' or 'frequently occurring' not necessarily 'popular'.
My OCD kicks in because it's not in order, but at the same time I like the chaos :)