Skin overlays that adapt to the skin color?

Hey y'all! Hope you're doing well! I've decided I want to try my hand at making a skin overlay. Thing is, I'm completely clueless as to how they make it able to be an overlay and adapt to the color below it? I can understand how to make a standalone skin that doesn't rely on the skin tone but I want one that can work on every skin tone. Any help would be very much appreciated!