Honestly, it’s just many many layers (with the blending mode set to ‘color’) of various colors. I’ve learned that certain parts of the face generally require different colors. Parts of the skin that are directly over bone (brow line, cheek bones, jaw, etc) tend to have more yellow, while the nose and ears tend to be pink. Another thing I’ve found helpful is dropping in a layer of dark red, like a maroon, towards the end and just painting it in over the entire face. I also tend to add a curves layer to help with the contrast of the image, as well as a hue/saturation adjustment layer where I pump up the saturation by about 15-25 depending on the image. I hope this helps you out.