Any skin you do, you're going to have to start from scratch. You can't just paint on top of an existing skin. For one, it will look bad, for two, it's like stealing somebody else's work and passing it off as your own.
You're going to have to work with layers, and that means you're going to have to do most of the work yourself, no matter what colors you want to start with. So might as well just do it the proper way (the way that will look best in the end) and start from the default and build up from there.
