While the painter Lucian Freud is known for his splotchy skintones, if you're wanting smooth skintones, glazing over the whole figure when you're just about finished painting will produce this.
Painting Forum Host and portrait painter Tina Jones say she paints "a translucent layer of white (either really thin titanium or zinc white) all over, sometimes more than one layer." This is followed by a glaze of red and yellow. Together these smooth the skin tones and integrate any splotches of color with the rest of the skin.
The photos show a figure painting by Jeff Watts reworked by glazing over with "the lightest of the skin tones and sometimes the shadow colors too." (The painting was first shown on the Painting Forum.)
A blue can also help pull the skin tones together, as well as red and yellow. Which you use depends on what is already dominating the skin. Another option is to glaze with either secondary colors (mixed or from a tube). Tina says: "sometimes cadmium orange or ultramarine violet will finish a work like nothing else. I'll even do glaze with the secondaries plus very little white. I'm a double timer sometimes at glazing, even though ideally one color at a time makes the most of it. If my figure is looking jaundiced, I create a lavender glaze from titanium and ultramarine violet to get them out of the bilirubin box and back on their feet."
With oil paint, glaze with paint thinned with a medium only if you've been using a lot of medium in the underlayers (remembering the fat over lean rule). Otherwise, use dry brushing to put a thin layer of paint down.
Tina says: "A filbert is a good brush for dry brushing. Scrub the paint over the top like a see-through cloud or thin veil. Be sure that the underlayers are dry so you don't blend what you already have there."
