This small painting, 20x28", was done in six hours with a direct approach, then, when dry, I glazed over the dark areas with some darker tones to beef up the background coloring and add more depth.
Here you can barely see my usual pencil drawing, lightly fixed, then washed over with a quick reddish-brown for the general skintone middle value. This wash, done with mineral spirits and a little liquin medium added to a small amount of paint color, dried in about 4 hours, but I let it set overnight. I usually do several paintings at the same time, so this was just one where I washed in the same reddish-brown base over all the paintings and let them dry throughly. It saves a lot of time and allows me to think far enough ahead that I can plan several pieces to evolve at about the same rate, if I know I will be painting for a few weeks straight.
Below, you can see my basic palette setup- 3/8" glass palette around 18x30", which I bought at a glass store and had them sand down the edges so they were no longer sharp. I place this palette right in front of my easel now, which was a big step in getting more accurate color. Since the light on the painting and the falling on the palette are the same, the color matching is a bit more accurate than if the palette is off to the side. It also keeps me from having to turn to the side every time and mix paint.
That little blue handled holder is for single-edged razor blades that make it much easier to scrap off dried paint from the glass surface. I try to get the paint wiped off before the end of every painting session, but if its dried or old paint, the blade will scrape glass much better than on a wood palette. I also have a traditional shape round palette made of glass, but rarely use it as I spend most of my painting time sitting now, or standing in front of the easel (studio painting) and do little location work.
I know a lot of artists who freehand their figures in and go for a more painterly look with less realism but more of an arty feel. For me, I prefer very accurate figures. I can do it either way, but after a lot of experimenting, I do like having a loose, or tight, outline drawing down first, one I have already sketched out smaller then projected onto the canvas. Sometimes I freehand this drawing as well, but the main idea is to get the proportions accurate without putting a lot of paint on first that I have to work around. In this way, I can not only work faster, but can keep the edges clean and my mind free to just worry about color and value.
At this point, I have smoothed over the background, which is a good idea to do as this nearly always looks better, aesthetically and for the design. If you use large brushes to block in the areas, as I have done here, then smoothing the paint is a simple matter of hitting edges with a soft, wide brush and dragging it over here and there. I can always add more paint and can sharpen an edge again by using flat stroke into the slightly blurred mass of color.
Normally I would do the face first, setting the mood for the piece, but this is a quick study so I feel pretty confident I can just knock out the figure and bring it forward. My only real concern is the value of the skin tone in relationship to the dark background, but now that the background is in, its easy to evaluate. Actually, this piece is somewhat ideal in that it has a soft background and simple foreground, simple coloring and no real pattern or texture to finesse. The figure should pop since its light skin on a dark background and the light on her skin is simple and direct. You can see why its a one day piece and I think why a lot of early portraits were done with dark backgrounds against well lit skint ones. Plus, it always has a nice overall look and suggests something classic in the approach.
I treat the skin the same way as anything else. Light falls on it in space- it has both reflective and a little translucent coloring and even if my color is off, it's the edges that will determine the realism. I could make her purple or green and still make her look realistic, even photographic, if the values are close and the edges are fused accurately.
As a general rule- edges (where two or more visual planes connect, as in the model's back and the dark of the background) can have a softer effect (blended into one another) when similar middle tones come together, highlights and shadows come together, or soft forms (like hair or fur) are seen. Also, look for rounding, or where light falls off a rounded form (like the womans legs or arms) and falls into a shadow area or changes coloring (usually from warm to cool, or cool to warm). Reflective surfaces can also help change color tone, like how the upper arm picks up an inner light from the skin itself. All these things add up to the physics of what makes things look and seem real. Since artists have to push this whole idea, working on a flat-plane, it's important to keep in your head, but essentially it comes down to really looking or studying what you are seeing before you, and attempting to match it on the canvas. For my more creative work, where I have to invent similar visual images, I can use these same ideas, of physics, to help achieve a realistic look even with made-up areas. This is especially important with illustration work.
There is not much difference between the top image here and the detail from the final, below, except for a few hours time and applying solid, direct painting without fussing with it too much. I know it looks like a lot of extra work, but the skin is just blocked in and then modeled a bit here and there with a bit smaller flats to push or pull the wet paint into the next areas. If you put down accurate color right off, then the fudging, or finessing of those strokes, is minimal, and the painting goes much faster. The accuracy and realism comes from drawing (which I solved with my initial pencil sketch on canvas) and then redrawing, if needed, in color with smaller strokes. I rarely work with brushes smaller than 1/4" flats, as the painting will get too fussy looking and I really want the look for my work to have a feeling of realism but also an illustrative stroke. You can see some detailing in the hands, but again, these were done with 1/4" flats (long synthetic brushes, in this case) and when I need more of a line, I just use the end of the brush or side and drag it across.
After the painting dried for a few days, I did float a dark glaze over the background and foreground areas to give them more depth. Once the main values are in, and I mean values, not color, you can easily coat just about any area of a painting with a darker layer if you float the color on then buff it out. It takes some practice but really helps to enrich the work and give it more of a sense of three dimensional space. I could have also enhanced the models skin tone, but left it alone. Just darkening the area around her seemed to be enough.