This is the time to really nail the likeness. Sometimes I tape my reference photo next to the area I'm painting.
However, you must be careful with tape on the surface of the painting. The surface must be dry. I stick the tape to material first before resticking it to the canvas in order to reduce the adhesion. I do not leave tape sticking to the surface of a painting when I leave the easel.
I begin by adding a glaze. I use raw umber, raw sienna, and burnt umber either alone or in combination - however I can to begin to create the sitter's skin tone. I may use a thin glaze of raw umber to draw details (like eyelids) and enhance the form (above).
I glaze in the skintones very slowly and thinly. I let each layer dry before beginning the next.
I brush color into the cheeks and begin to define eye and hair color.
Note that if this was a woman of color I would begin with a darker value of skintone in the underpainting - the rest is exactly the same.
I am adding color slowly in layers of glazes. Then I am painting tiny bits of Titanium White + Yellow Ochre Pale into the wet glazes to build light.
Light is thick and opaque.
Shadow is thin and transparent.
I am mindful of the Old Master's technique of color banding:
Yellow Ochre Pale is added to the forehead.
Reds (Indian Red and/or Alizarin Crimson Permanent) are added to the area from the eyebrow line to the chin.
A cool blue (French Ultramarine) is added to the neck and chest.
If you mix these up - the face will look really odd; i.e., red on the forehead will make the face look ill - like a boiled lobster.
A glaze is a lot of medium (I use Liquin) with a little bit of transparent color. It will look a lot like pale stained glass.
Sometimes I scumble a color on the face to even out the tones and add luminosity. A scumble is a whole lot of medium (Liquin) with a little bit of opaque color (like Titanium White). It often looks like watered down milk.
After the underpainting in white, all of my layers are transparent or translucent. I already had a likeness so I didn't want to lose it.
As I am doing the final glazes and scumbles on the face and hands, I am glazing other colors in the painting to enrich the colors.
Note that because of my earth palette, I don't use the color "blue." Instead I use Ivory Black + Titanium White to make the color blue. If at the end I want to intensify that color I can add a blue glaze if necessary.