Portrait Painting in black&white oil - PART II

This is part II of my attempt to paint a black&white portrait as shown in the book "Creative Illustration" by Andrew Loomis.

In part I I finished the charcoal underdrawing.  In this post I'll show some photo's of the beginning of the painting stage.

The black paint I used is a mixture of Ivory black (a blue and cool black) and Burnt Umber. I pre-mixed four values: An almost white gray, a light grey, a dark gray, and an almost black gray.

1. I started blocking in the darkest parts and the background.

2. Adding the whites and a light gray for the skin.

3. Adding more details on the hair and face.

I felt this was a critical moment in the painting, how to continue?
You can see how the painting turned out in the upcoming part III.