Before Yahoo's Open-NSFW data was released, skin tone analysis and assessing if the image contained a majority of color palettes that are generally considered to correlate to flesh tones. You'd be surprised to know that many of these libraries are still commonly used (look at nude.js for example). The downside is that they are false positive heavy, and fairly easy to circumvent.

What did not having a crayon in a particular colour teach you?

Your best options for standard crayon sets are probably pale-skinned and sunburnt (bright pink) or somewhere around chocolate brown.

Most sets have pink, white, black, brown, and yellow so most people should be able to do a close average tone to their facial colour?

This was a colouring-in by numbers book so they had a particular one in mind.

I don't get it, you had a colouring by numbers book with your own picture in? Sorry, I'm missing something, ELI5?

Well, because "flesh" is not skin in the first place.

I assume the book was written/printed in the UK where "flesh" and sent out to us colonials. This was the 60s. "flesh toned" is pretty ordinary term for "skin tone" even in the US I believe.