Great question. If I understand it correctly, I could reword it like this: “How important is sourcing photos of a specific person’s skin for a likeness portrait of that person?”
I would try to source photos of the target person initially. If you’re doing a celebrity / well-known person, chances are high that you will be able to find decent photos online. If you or someone you know is your target, I’d recommend taking outdoor photos on an overcast day. That will produce minimal shadowing.
If for some reason neither of these apply, I completely affirm what @melinmotion said: Just find photos of a skin tone that’s close to your target person(hopefully something on the provided reference sheets will suffice). In fact often I will chose this option if I can’t find good target photos quickly. Because skin tone is extremely flexible after it’s painted.
This is why I don’t put much emphasis on matching the skin texture to photos of the person. I used to search the internet for a long time for perfect photos that I could basically cut and paste on to my model of the target person; naively thinking that Tom Cruise’s skin was generally recognizable feature. But it’s not. Facial shape is the #1 goal in likeness. Skin texture is far more flexible when it comes to likeness.
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