You must thin down your paint color to make it flow on (washes) and quickly remove some of the color to leave a light layer with no hard edges (feathering). In folds and creases, paint the color wash in and then dry brush to feather the color out. On larger areas, pounce on (and off) with a mop type brush, cloth, paper or soft cosmetic sponge. This can be done several times to achieve the depth of color you desire.
Some flesh shadows are a pale pinkish color and some are red-toned brown. For example, a pretty fairy and a rugged cowboy would have very different applications of shadow color. What paint color you choose depends on the effect you want to achieve with your doll art.
Once you finish the shadow, you apply thin washes to add color where human skin is naturally reddish in tone (think make-up shades). You can add accent colors around the eyes, and for men, a darkish 5 o'clock shadow.
Most new doll makers are timid when applying color to their finished dolls or their paint is too thick. The best advice is to play around with color washes to learn how to use them, and how they look, on polymer clay.