Our app extension fixes to SimplePic went live last night (we solved the app store search bug of Pic! by just changing the name. Crude, but effective. The fix to app extension was replacing some of the Swift code with Objective-C... old technology has more warts but fewer bugs). I tested it on Tina Ziemek's tattoo as that is one of the things SimplePic is designed for:
Top: original image. Bottom two output of SimplePic
The middle picture is a good example of the tradeoff in changing colors without messing with skin tones too much. Some of the hues change a little as some of that tradeoff. There is most definitely a few SIGGRAPH papers to be had in this domain, and I am not going to pursue it, but if anybody does please keep me apprised on what you find (there is a correlation between tattoos and iPhones so there's a specialized app to be had as well). The posterization I personally use when I want to exaggerate contrast (like a photo of a sign where the colors are blah) so I wouldn't probably do that there.