Dear artists...
If your clients give you references with heavy lighting or ambient lighting and you cant seem to get the right skin tone...
THROW FLESH CUBES AT THEM AND HAVE THEM PICK ONE.
I've been using these for years and I've never had anyone tell me I've gotten their skin tone wrong.
When in doubt skin cubes.
I have no idea if the artist who made these has an X but their username at the time these were posted to deviant art was Puppsicle.