Here is a quick example of using more colors for coloring.
If was done in about 6-7 mins
I sampled colors from this image:
This is the original B&W image from google:
And this is the colorized version:
At the end i used selective color adjustment to fine tune red and yellow to the desired values.
Some parts could still have a little less yellow in them (highlights) and you would probbly want more cyan in the shadows than in other parts of the flesh tones
And keep in mind that hair/lips/teeth/eyes color is basicly a variation of the skin tone so it will look unnatural if you go with a completely different color
Hope this helps at least a little bit
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