Water colors are by far my favorite form of portrait art. I get lost in the airy strokes of my brush, as I run it across my toothed paper. I always find that creating skin tones can be the most difficult thing to do, when it comes to mixing water color paints, so I use both tubes of water color to mix my flesh tones, and a basic pre-mixed pallet for everything else.
I begin by making a light base tone for the skin. Not too dark, but just visible enough. I also try to get as many values in as I can, with my first strokes. It's always easier for me to build off those values, afterwards.