Below, one of the images of Whit from my book, showing how I evaluated my skin tone mixture by comparing it right against his skin.
Here is the studio set -up:
I took several in-process photos during our model breaks to show progress. Below, Step 1, placing and measuring to begin the drawing; separating light from shadow; and committing to the background.
Step 2: covering the canvas, roughing in skin tones in both light and shadow.