I did this the over last few days to try, and copy something from my favorite painter. It was my first master copy, but I got so much from it that I decided to do a series of copies of Rembrandt and other painters.
I cropped a little bit, as I was more interested in trying to get the background right (failed), and see if I could capture the likeness and skin tones precisely.
In retrospect, I can see lots of problems with my copy: the background is too dark, too warm, and it lacks the atmosphere the original has. I think he did it by scumbling a coolish gray over the brown wash in. Maybe he applied some medium to the dried surface before scumbling. I'll try that next time. The drawing is not exact, but I was happy with it. Close enough, I think.
I tried to do it the way scholars assume Rembrandt did it. He started with a toned ground, did the drawing with a dark transparent color directly on the canvas, getting the midtone and darks into place, then he'd lay in a gray tone over the shadows (according to Joseph Sheppard). He'd then add the dark accents, and start on the lightest parts.
The face on this one was heavily textured, and he'd supposedly get the texture first, and then correct the colors, sometimes glazing a dark tone over that so it would accumulate in the lowest parts. I didn't do any of that. Trying to get the temperature, levels and drawing right was hard enough.
Anyway, I was overall satisfied with the result, but what mattered most was the experience, which I intend to repeat again and again.
ps.: my palette was Flake White, Naples Yellow, Raw Sienna, Cad Red Light, Burnt Sienna, Burnt Umber, Ivory Black.