Yesterday, an artist friend of mine and I were discussing her recent attempt at doing a watercolor portrait. She showed me her work and it was surprisingly good for a first effort. She is no stranger to painting portraits or watercolor either but we both had to admit she has a ways to go in learning to merge the two. Watercolor portraits are probably the most challenging thing any painter can do. When I was in college (a senior) I did an honors paper on portraiture (comparing various methods, sources, and media) and several of them were done in watercolor. It was good in that it prepared me later for working in colored pencil because, of the similarities in handling color.
With watercolor portraits I first begin with a low-contrast, but quite accurate, pencil drawing (from life or photo, doesn't matter, even a combination of the two). Then I complete the background, hair, clothes, and smaller, usually simpler, areas of fleshtones (arms, hands, the neck, etc.). I also paint in the mouth, eyes, and nostrils (all not very different from working in oils/acrylics). Once the rest of the painting is dry, I begin the fleshtones by first wetting the entire facial surface with plain water (no puddles). Then, I begin with a layer of yellow ochre over all but the very strongest highlights of the fleshtones with more intensity in the shaded areas. Then, maintaining wetness, I moved to a cadmium red light and worked over most of the same area, concentrating it in the darker areas of the face. The trick is to work quickly and keep the entire facial area WET at all times. This avoids tendency of watercolor to form hard edges where you don't want them. Following that, I introduced cerulean blue into the darker areas where needed to cool off the other colors, followed by a little umber (or if I'm really brave, sometimes thalo blue or green) in a few of the very darkest areas. This is highly simplified. It's not as easy as I make it sound.
As all watercolorists know, the medium demands "looseness" and usually looks better when handled that way. Portraits, on the other hand, are such a demanding endeavor they tend to pull the artist in the other direction, causing him or her to "tighten up." In LEARNING to do watercolor portraits, I think this is good. In watercolor portraiture, you've got to maintain a certain "tightness" in order to turn out a good likeness. And most of all, you've got to NOT think like an oil painter in handling the paint. I suppose this is why you see so very few watercolorist doing portraits, and of those, so few that are really top-notch. Watercolorists LOVE the looseness of their mature painting technique and especially if they've painted portraits in oils, find it quite difficult to make the transition to a medium demanding the antithesis of their traditional watercolor AND their oil painting portrait experiences.
So, having said that, the key to success in doing watercolor portraits therefore becomes SPEED! You have to keep that face wet (especially with younger, smooth-faced subjects) so therefore, every instinct of oil portrait painting technique with regard to handling fleshtones (what I call "lovingly lingering"), must go out the window. Transparent watercolor is a glazing technique not unlike doing so in oils, but with the timeline speeded up by a factor of about a million to one. Drying takes seconds instead of days. You are literally "wrestling" with the elements to keep your likeness, the colors, the blending, the edges, the bleeding, AND the values ALL under control--which means any "looseness" is an accident (and usually not a pleasant one), at least until all these control problems are mastered, at which time I think the loosening up will come naturally.
Add to this, the fact that watercolor is a VERY unforgiving medium and you have a real challenge. Unlike oils, where virtually EVERY mistake can be corrected (given the time, persistence, and know-how), there is practically NO point along the line in doing a watercolor portrait where anything more than a modest error will not RUIN the work permanently. Frightening isn't it?