Exactly what colors you use for painting skin tones and how many is a matter of personal preference and style. About the only thing that's certain is that having one or two tubes of paint labeled "skin color" (the names depend on the manufacturer) isn't going to suffice.
The paint shown in the photo is a tube of "Light Portrait Pink" acrylic, produced by Utrecht. It's a mixture of three pigments: naphthol red AS PR188, benzimdazolone orange PO36 and titanium white PW5. I've had it about 15 years and as you can see, I've used only a smidgen. I find it too pink to be useful for any skin tone, even when mixed with other colors. Perhaps one day I'll use it for a pink sunset painting?
My preferred colors for mixing the full range of skin tones are:
- Titanium white (with watercolor, the paper acts as the white)
- Titanium buff
- Cadmium yellow (medium or dark)
- Cadmium red
- Burnt sienna
- Raw umber
- Prussian blue
- Payne's grey (not essential, but useful)
If you don't like using cadmium pigments, substitute whichever red and yellow is your favorite. The advantages of cadmium red and yellow are that they're both warm colors and have very strong tinting strength (so a little goes a long way). It's well worth experimenting with all the red and yellows you have, to see the results you get.
The blue can be whichever you prefer too. I like Prussian blue because it's so dark when it's used thickly, yet very transparent when used thinly.
These are certainly not the only options open to you. Everyone develops their personal preference through time. Experiment with golden ochers, deep purples, ultramarine blue, and greens. Pay attention to the underlying color of your model's skin too (not their dominant skin tone). Is it warm or cool red, blueish, cool or warm yellow, golden ochre, or what? If you have trouble seeing this, have a look at the color of various people's palms and compare theirs to yours.
Color mixing tip: a little of a darker color mixed into a lighter has a much greater impact than the same quantity of light mixed into a dark. For example, umber added to yellow rather than yellow to umber.