Here's a great skin tone painting tutorial that's applicable to custom figures.
A course by Andrea Miniatures, this is a tutorial of how to paint eyes and layer the skin tones on a miniature. This also includes the 5 O'clock beard effect that brings out the realism.
While most 'comic book' style characters don't have as many gradient
steps, some custom figures (particular NECA-style, or realistic
depictions of people) benefit from this approach. I this approach (although not the same colors) when painting the face/beard on my Hellblazer: Constantine figure
(Simon Bisley styled version).
I've also edited together two infographics for easy reference from video screencaps:
