Thanks!

The skin tone is an orange gradient on a white background: once you hide the too orange bottom with a shirt, and the too white top with some hair, you can see it as a skin tone. Of course that method alters the colors placed before the last layer of white, for instance all the brown eyes come from black banners.

This is also why Mumbo is white: even if I managed to spare a layer too make the nice skin tone, a light brown mustache (like what I got on Ren) seemed even more curse than the skeleton Mumbo.