To change skin color of a character you need to edit the palettes for every character, like this:
If you check, there is one gradient that sets skin color. You need to change every pallet from body and head to set the skin color you may want.
In fact, it is not as hard as it sounds, but only a very huge load of work for anybody who may want to try it. The hard thing is to get the exact color for skin, since can exist multiple skin colours.