A secret formula to get skin tones right? Sounds like a complete joke at first. But then, I discovered this video by Sean Tucker.
And it all seems to make sense. At least, it looks highly interesting and worth checking out.
Conclusion
There are formulas that describe naturally pleasing skin tone values. They are expressed as relative proportions of cyan, magenta, yellow and black. First find the cyan value. The magenta value should be double of cyan. Yellow should be one-fifth to one-third higher than magenta. Don't convert your image to CMYK and ignore black values.