How to Paint Shadows on Faces in Oil

So the light source is pretty obvious, streaming across the right side of the face.  Where the light hits an object, it is sharper, with more colour.  As it fades away from the light, the shadows are muted in both colour and sharpness.  This is where I add my purple mix to my basic colour mix using Ultramarine Blue.

Behind the glasses, the eyes are a little softer, and on the shadow side of the face, can be a little more purple to suggest the lens.