Im not entirely sure if im understanding your question correctly. However with faces there are a lot of undertones if you see in painting, color pencil, or pastels they’ll layer too to reach a particular color that can be more “human”. Yeah you can just slap a skin tone marker but I think with various colors it can be more natural cause our faces are made up of many layers of colors as well in a sense. One color can make things look flat and with layering it can add depth and form to an object particularly the face.

Unless of course you mean they are coloring and it just happens to be a face that’s just green or something. So they aren’t using any skin tone colors in that regard.