Just about any combination of red, yellow, and blue will produce a flesh tone in watercolor. Each situation is different and no formula substitutes for your own observation. I will share what works for me.
For average caucasian skin, I use Winsor & Newton Raw Sienna or Yellow Ochre (they're quite similar), M. Graham Cadmium Red Light, and a blue, violet, or (rarely) green, The blue depends on the subject, the lighting, and the pigments I am using in the rest of the painting. Cerulean blue is probably the easiest to work with and works best for light-skinned subjects. Sometimes I use Winsor & Newton Cobalt Blue and sometimes I use Holbein Mineral Violet.
When working with fair complexions, I move toward a cooler yellow such as Winsor & Newton Cadmium Yellow Pale. As complexions get darker, I move to darker yellows and earth tones, stepping down from Raw Sienna to a brown ochre to raw umber and finally to burnt umber. An example of how to paint darker skin tones is available.
Reds get warmer as complexions get ruddier. For a young, fair-skinned model I might go with a permanent rose. I will choose a warmer red, such as cadmium red light, for skin that is darker or has been exposed to the sun. For really dark skin, I will choose anything from cadmium red light to even a cadmium orange in some circumstances.
The cool color, used mostly for shadows, is the most difficult to categorize because so much depends on the light source. For shadows I use the same yellow and red that I used in the light, and I add a cool color that neutralizes the yellow and red. The most important thing is to not use too much blue in the shadows. You basically want a darker and more neutral version of what you have in the light.