Firstly, if you have ever bought a tube of flesh tone or skin colour, throw it in the rubbish now. The idea that one can obtain skin colour from a single paint tube is ridiculous and any manufacturer worth their product should not produce it. The colour is manifestly and overtly pink nor is it colour fast and lacks permanence.
The colours you will need for skin tone are: Titanium White, Naples Yellow, Cadmium Yellow Deep, Yellow Ochre, Scarlet Lake or Vermillion, Venetian or Indian Red, Burnt Sienna, Raw Umber or Van Dyke Brown, Prussian Blue. There is no one formula for skin tone. As you will see in the image on the right I keep a record of my skin tone mixes as they vary greatly but each of these has been used in a different artwork. The burnt orange on the top left was used for Shelly's Hallucination while below it the more pinkish was used for Girl28 and so on.
Skin colour will be dependant on not only racial differences but also in the setting of your figure and surrounding colours of your work. The painting style will also have an effect whether it be the pearly radiance of an Ingres or the warm glow of a Renoir. To this end you will need to experiment yourself to find the right combination for your work and what you are trying to achieve. In general I start by mixing a little Naples Yellow to my white as this gives a creamy base to work from. Using pure white in skin tones should be left only to small highlights if at all. Into this creamy white I will add combinations of my warm reds and browns and perhaps some extra yellow until I have my middle tone for the work. I ensure I have enough middle tone to last me the whole painting as trying to remix it exactly if I run out can be a major headache because every subtle difference will show. I wrap spare middle tone in silver foil or keep my palette covered in silver foil to protect it from drying out. You will soon realise the benefit of having extra middle tone if you need to rework something later that you are not happy with.
Many artists will start by mixing a light, middle and dark version of their skin tone and apply this to the painting, blending all three as they work. However this can often lead to an overall mannequin like colouring of a nude figure and lacks the subtle differences needed to achieve good results. Also this implies that the figure is painted as one layer which causes a flatness to the work. A nude or portrait should be painted in layers combining opaque areas and translucent washes to build up depth and radiance. Prussian Blue is a very intense colour and can be used in very minute amounts to cool the skin and provide depth in areas around the eyes, between the fingers, under arms etc, remembering that blue will receed in relation to red. Study Master paintings to see where blue is often used in nudes.
Scarlet Lake or Vermillion can also be used not just to form your middle tone but also by using small pure amounts to blush the cheeks, elbows, knees or anywhere to show warmth. Indian red is good for lips, nipples and for adding warmth in shadow areas. Do not be afraid to add tiny amounts of pure colour as you work, a little cadmium yellow deep can soften the transitions from shadow to mid-tone or some Prussian Blue provides extra depth to shadow areas as well as highlight areas.
Above all look to your art books for guidance from the Masters or better still visit your National Art Gallery. Take with you a notebook so as you study masterworks first hand you can write down what you see and make small sketches of how it was achieved.
Tip: My personal preference is to refrain from using Burnt Umber which lacks warmth and still results in mud even when blended with warmer hues. Instead darken Burnt Sienna with Prussian Blue and or Raw Umber. If you need very dark shadows wait for your work to dry and apply a glaze of Vandyke Brown or Raw Umber. Feather the edges with a dry brush.