It's pretty clear from the original chapter that Mumu's skin is on the darker side, while Youta is light-skinned. I really don't know how anyone could come to a different conclusion.
@ben3factor to answer your question why is Mumu's skin is white on my version, it's because I based her coloring on the only colored picture I could find of her. In hindsight it would probably have been better to stick with the tones present in the manga but I already finished a couple of panels by then so I rolled with it.
That pattern circling her arm... It looks like it's a string of fabric that has been pierced through her skin multiple times so that it's half the time inside the skin, half the time outside.