After watching the complete Video I find this approach very useful for quick and quite good results. His formula in CMYK space:
C = the starting point
M = Cx2
Y = Cx1.25
K = not relevant
After a quick research on the web I found other skin color ratios for various color tints:
Coming back to darktable: The color picker supports RGB and LAB.
If someone would be able to convert these ratios into LAB space we could use the tone curve to adjust skin colors.
I don’t remember that darktable uses CMYK in a module (correct me if i’m wrong)