There are any number of ways to mix useful colors to represent flesh tones. A set of flesh mixtures needs to be able to harmonize across light and shadow zones and represent different local flesh colors within a single person. It also needs to work well for people of different races and complexions.
Lately, I've been working with a flesh palette composed entirely of earth pigments (plus white). Here are the oil colors I currently use:
1. Flake/zinc white (the only non-earth color in this palette). I like Doak's flake #1c. Williamsburg silver white (another flake/zinc combo) is also nice, but kind of thick.
2. Raw sienna. This is Williamsburg Italian raw sienna.
3. Burnt sienna. Williamsburg. This is an earth red, semi-transparent and slightly violet in tone.
4. Red ochre, Williamsburg. Another earth red; this one is more opaque and a more neutral red.
5. Tuscan red. Studio Products. A violet-red, semi-opaque. Good for toning lips and cheeks.
6. Ultramarine blue, Studio Products. I call this an earth color because ultramarine blue pigment was originally extracted from lapis lazuli. Modern ultramarine is synthetic (and much cheaper) but chemically identical.
7. Vermllion, Doak. This is genuine vermillion (mercury sulfide), a bright orange red. It's very strong and only a tiny amount is needed in mixtures. I call this an earth color because it is a synthetic version of the mineral pigment cinnabar. Vermillion is particularly wonderful because, when mixed with white or broken with other colors, it retains much of its chroma. Cadmium reds of similar hue, by comparison, become dull in mixtures.
8. Terre verte, Williamsburg. This is the brightest, cleanest, and coolest green earth I've encountered. It is much stronger than other green earths, even though the company says that it hasn't been enhanced with any other green pigments.
9. German earth, Williamsburg. This is an earth black.
10. Yellow ochre, Williamsburg. A basic dull yellow.
11. Yellow ochre extra pale, Doak. This is lighter in tone and higher in chroma than regular yellow ochre.
12. Raw umber. Williamsburg.
13. Burnt umber. Williamsburg.
This is more colors than strictly necessary, but the variety is convenient, since you don't need to do quite as much mixing. (Also, I'm a sucker for beautiful earth pigments.) This palette is not flexible enough for most sorts of general painting (although you can do a heck of a lot with it), but for realistic flesh tones you can easily get pretty much any color you need.
I painted the self portrait posted earlier with this palette. Caucasian flesh is best painted with a mixed orange base, for which I use a blend of red ochre and yellow ochre. I mix a string of this color with white to mix a series of values. I then mix a string of darker values by adding raw umber to the ochre mix. I set up a neutral string of raw sienna and ultramarine mixed with different amounts of white. Sometimes, I have an additional neutral string of burnt sienna and terre verte or raw umber and German earth. For most purposes, the ochre mix is too high in chroma, so I mix it with one of the neutral mixes of the same value to get the paint that I will apply to the painting.
To adjust the hue of darker values, I add any combination of ultramarine, German earth, raw umber, burnt umber, or terre verte. I can lighten with white without loosing too much chroma and getting a "chalky" appearance. Areas of skin that receive more blood, like the ears, cheeks, and nose, can be reddened (or glazed) with vermillion, burnt sienna, red ochre, or Tuscan red. More sallow areas can be toned with either of the ochres. African skin tones can be mixed by adding umber, ultramarine, or terre verte to the yellow ochre + red ochre mix. Asian skin tones can be made by adjusting the proportions of the base ochre mix.
If you like to paint in multiple layers, you can do a cool underpainting with ultramarine, terre verte, or a traditional Italian neutral "verdaccio" (which traditional Renaissance Italians mixed from white, black, yellow ochre, and red ochre and applied over a flat terre verte base). Allow to dry, and then glaze with appropriate colors, with transitional tones and darks blending with the underpainting to create interesting tones that can't be created by direct mixing.
Earth colors tend to naturally harmonize with each other. They also have the advantage that they are generally low in chroma (intensity). Skin tones are much lower in chroma than most tube colors, so if you are working with more chromatic colors, you need to do a lot of work to get accurate tones. Many artists have trouble mixing accurate skin tones, because they have difficulty neutralizing the high chroma colors on their palettes. Most flesh is really low in chroma—near neutral—and you see a lot of amateur (and some professional) work in which flesh tones look wierdly chromatic.
You'll note that I use a lot of earths by Williamsburg. They make a nice range of high-quality oil paints, with an extraordinarily good set of earth colors. The consistency is a little thicker than I prefer, but otherwise they are excellent. I mentioned Robert Doak's paint earlier, and Studio Products paint is also top of the line.