Joaquin Alberto Vargus (1916-1986) did all his works using either watercolor or an airbrush or a combination of both.  A lot of his works were favorites during WW2-- painted on bombers etc.
He was considered a master using the airbrush -

In Photoshop or PaintShop Pro, you trace out a picture with the black pen, in your next layer, you paint a basic skin color, next layer, skin tones, third layer black shadows (you can adjust the transparency), next several  layers, your light areas, save it and merge everything, then use the smudge tool and blend the areas together, do the same for the eyes and lips and teeth.  Put high light areas in and blend them too.  Is it easy, heck no and it takes several hours or days to get it right.

Jan