To reinforce what @johnny-bit said, please reread my post.
The problem with this is that many papers still use the most egregious of colour spaces or models as their basis for skin detection, requiring hacks upon hacks to compensate for their deficiencies. Either that or they rely on machine learning. There is no in between in the “science”.
That said, video editors often include a vector scope. Often, people use its I line to ballpark skin tones.
PS Forgot to mention that you have to be careful about CMY. It is another colour model that CMYK is related to, so they are different things. Unless by CMY, you really mean CMYK.