Richard Schmid, in his book Alla Prima II, Everything I Know About Painting and More, describes a reductive technique for doing an oil underpainting. It's reductive because you begin by painting a midtone single color (monochrome) over the entire canvas, then define the light shapes by wiping paint away with your finger in a cotton rag. In this manner you start with defining shapes instead of drawing lines, and you focus first on the light shapes instead of the dark shapes. You add more paint as needed for the dark areas as the figure begins to become defined.
For the monochrome color I used a mixture of equal parts transparent oxide brown, transparent oxide red, and cobalt blue light (as Schmid recommends). In Schmid’s technique, as I did here, no white paint is used. You achieve the range of values by building up thicker paint with a brush for the dark areas or removing paint for the light areas. The lightest lights are achieved by rubbing away the paint all the way down to the primed surface. In Schmid’s technique, you also use no medium or additional oil – just paint straight from the tube. Each of the paintings below, I completed in a single session while all of the paint was still wet.
These are meant to be underpaintings, meaning they allow you to establish composition and light and dark values without worrying about color. Then when the underpainting is dry, you would do a color painting on top of it. In Schmid's technique, the underpainting is sometimes so well done that it can stand on its own as a completed painting in monochrome.
For the first painting below, I copied the example in Schmid's book. For the second figure below, Mandy, I painted on my own from a reference photo, in a single session using the reductive technique.