One of the most distinctive things about an artist's portraits is the way he handles fleshtones. My first fleshtones consisted of white, burnt siena, and burnt umber. So long as I kept the burnt sienna between the white and the burnt umber, the results were adequate for a beginner though the effects are quite limited. I used this combination until I was in college and was practically FORCED by an instructor to use something other than earth tones. Then I settled on white, yellow ochre, cadmium red light, and ceruleun blue, a combination I still use today with occassional variations. I've seen portrait artist use veridian with alizarine, dioxine purple with yellows, pthalo blue with oranges, etc. There are practically an infinite number of combinations and probably the best combination is to use NO single formula at all. However basicaly, the three primary colors, mixed in variying amounts, create various shades of brown, which, when used with white highlights, create fleshtones. Likewise, a primary and it's complement (across the color wheel) will work too, along with white of course.
My suggestion for artists wanting to wet their feet in portrait paint is: DON'T. No, not because we portrait painters don't need the competition. It's just that there is too much to learn about portraits to tackle them in paint right off the bat. Learn to draw them in ordinary pencil first. Sketch them out from good photos with a #3 pencil then go over them again with a softer pencil (Like an Eberhardt Ebony). The one thing you don't ever want to have to do in painting a portrait is to struggle with the drawing. After you've mastered the black and white pencil portrait move on to colored pencil. Dry color media is MUCH more forgiving than paint and as a beginner you'll need all the forgiveness you can get. Colored pencil has the advantage of mixing in layers much the same way as paint but is up to 50% eraseable with NO drying time (a lot cheaper too).