I learned this technique some 6 or 7 years ago in some short Photoshop course and totally forgot about it.
There are different combinations of CMYK percentages that represent common variations of proper skintones from various races.
Of course, no skintone is exactly the same so this is used as a guideline and you adjust the percentages just a little bit for your actual subject.
Then you can throw that corrected still into your editing suite and use it as a reference.
Here is tutorial link:
http://www.graphicconnectionkc.com/skin-tone-correction.html
At the bottom of that page are most common skintones with CMYK percentages.