Skin tone is usually a mixture of red, yellow, blue and even green, and the exact color depends on the skin tone really. Usually lemon, cadmium or ochre yellow, cadmium red, cobalt or cerulean blue, hooker's green (be careful about this one) and even raw sienna and raw umber as well. Shading is done with the darker tones, ie blues and greens and darker browns and always layer and pay attention to transparency of the skin. Hope this helps.
It's not really a science or simple recipe for me - I look at my source and then mix colors on my palette to match, applying them in layers, diluting with more or less water, until I get the desired effect.