I usually start my portraits by sketching with a thin layer of oil color and solvent, and a tiny bit of linseed oil. This dry brush layer is translucent and easy to wipe off for needed corrections. Anderson uses a neutral mix, but I find myself gravitating to a warmer pigment in the underpainting. Beneath all our varied melanin producing skin cells our human flesh is red, pulsing with life giving blood. Lately I've favored Gamblin's Transparent Earth Red for this sketching under-layer of my painted skin tones. Red ochre has long been used by artists to depict the magic of being alive. The Gamblin color is a synthetic red oxide; I enjoy its greater vibrancy as compared to its ochre-based counterparts like burnt sienna or yellow ochre.