Hi, me again. Wow, I think you've got a great thing going with regard to drawing with your color. This is a great big improvement over sketching your shadows with black pencil or charcoal first, and painting over them.
I'm seeing hints of where you used different tints of color for your skin tones, too. It's funny, Nimoy on-screen is often made up using a colder color that tones down his natural, warmer (human) skin tone; but for publicity shots such as this one, he doesn't go through the full treatment. (It was the reverse for Zachary Quinto, who got caked with green powder for the publicity shots but not the movie.) There was even a Newsweek cover once without Spock's trademark bangs, just his hair combed over to one side like any normal human. Sometimes the makeup can be so inconsistent that it's hard to decide which skin tones to use, but I think you're correct with staying closer to yellow/olive than pink.
However, you do need to take chances with warmer reds, particularly 1) just inside the concave areas of his cheeks, where they're sunken in a bit; 2) on his forehead, in the middle zones between the chalky lights and the darks; 3) his lips. Yes, Nimoy wore pink lipstick (in TMP he wore too much, in TWOK too little), as do most men acting on-screen under heavy lights.
One more big, big suggestion: When you're looking at a photograph, you need to pay attention to the lighting of the room where the subject is positioned. In this photo, Nimoy was probably pictured in front of an ordinary black velvet cloth. That reflects no light at all, the result being that the sides of his face fade into pure darkness. He's only being lit by a warm light from straight on, and a weak, cold light from above, otherwise we'd never see where his head stops and the curtain begins.
You're trying to recreate the same layout of shadow in your portrait. But you're leaving the background white, the result being that the painted part of the portrait looks cut out from another piece of paper. If your subject were seated in front of a white cloth, the bounce light from that cloth would be reflected in his hair, the sides of his neck, his ears, and to a lesser degree his shoulders. If it's your intention to leave the background white for the sake of your theme, then what you need to do is "reinvent" this bounce light, knowing that the very edges of his head would not be jet-black.
It's getting better and better each time, Armus. Keep up the evolution!
DF "For Your Information, Captain, It's Called 'Colour Riche Pink Tranquilo.' I Have an Extra Tube If You're Interested" Scott