When I paint portraits, I invariably start with a direct sketch on the canvas using a brush and thinned paint. I just use a lot of OMS in the mix. That keeps the chroma a bit subdued and it causes the sketched lines to dry fast. They are therefore less likely to mix into the skin tones. The color varies, usually depending on the overall skin tone, but it's usually some variation of transparent oxide red or burnt sienna. Sometimes a gray is appropriate. The brush is generally a small bristle filbert, but I also like the sharp edges of a very small flat. (They quickly become 'filberts.' I'm a bit rough on brushes.)
Once the major facial landmarks are located, I typically establish the darkest darks and then begin to move to halftones and then lights in color. In other words, I begin painting right away. However, occasionally I take the first block in further using light and dark shades of the sketch color only. I just love these block-ins, and frankly, I'm often more satisfied with them than I am with the resulting completed painting. (I'm aware of the classical technique of grisaille, and I guess this is a version of that....but I don't think of it as an under-painting for glazes to follow, but just a 'black and white' sketch that will guide me as I paint -mostly opaquely- over it.) Here's The Preacher in the first sketch. I've gone past this point already and I'm well into the fully painted portrait. In this photo I see some of the initial drawing errors that I hope I have corrected in the more complete painting...but still I almost wish I had stopped here!
I'll post the final version when it's done. Still have a way to go.
Oil on linen Approximately 20 x 16
My Art Site: Bruce Hancock Fine Art