This is a hugely instructive color chart I did this morning that looks at glazing skin tones with different combinations of yellow, red and blue.
I used light washes, first of each yellow in my current palette, followed by the reds, and then two different glazes of each blue, one darker than the other. It's so interesting to see the subtle (but powerful!) differences in the colors, and I especially like how the chart illustrates the use of blue to "grey down" a color. Often my attempts at skin tones have been too yellow or too red, and I think the problem is not that the washes were too strong, but that the colors were not neutral enough.
I think this portrait I did today (based on a photograph by a photographer named Carla Tramullas) shows some improvement in managing skin tones.