A couple more points
One of the first things that I learned about representing skin tones is that the micro-texture of skin causes all kinds of exciting colors. These colors are most effectively represented (IMHO) via a pointalist or impressionist technique, the smaller the bits of color the better. This is why rough pastel work sometimes gives very believable vibrant flesh tones. There are real reds, even neons in there along with a range of blues and yellows. By representing these as pure dots of color and limiting the number of distinct colors, some of the vibrancy of the skin comes through. Another thing to remember is that highlights and cast shadows are going to tend to the opposite direction of the dominant light color. This means that in a warm light, the highlights and shadows will be cooler than the mid-tones. This is necessary to get the wider selection of wavelengths packed in there to indicate the brighter light - the reverse for the shadows.
As far as a general "formula" for skin tone when colors have to be flatter (e.g. cheap printing or painting small faces), the most common mistake is to leave out green in the formula. When that face just looks too dead or like a mask - try adding a little green in the mix of the mid-tones.
We don't know what we don't know, that is the beginning and end of it.