When painting the head you may hear simplify. What exactly, does that mean? One way to think about it is using a sculpture of the head created byJohn Asaro. His sculpture of the head simplifies the planes of the face.
For my painting below, I think about planes of the head. First I did the painting by drawing with paint. I paint I chose was burnt sienna. Whatever color you choose, make sure you have control the color. Phthalo and cadmium colors are very strong colors. A little of it goes a long way and it is easy to contaminate other colors. If you use a dark color and you think the final painting will be very light- then you may be wasting your time by constantly covering a color. So choose wisely and have a reason why you chose that color.
The middle image:
By using the reference of the Asaro head, I think about color as value. Notice that the color for the mandible (jaw line) is of different color than the cheek. The color is different but I am thinking about planes changes using color.
The right image:
I took a picture of my painting and changed it to black and white. When you work from a live model there are break times. Use that time to take a photo and see where you are. If you are working from a reference I suggest using a monitor not a print out. A print out photo is dye on paper. A computer monitor in comparison will be closer to a live model because it is emitting light. Whenever your computer goes to sleep - walk away! You want that fresh pair of eyes to see the value changes. Once you stare at something too long everything just blends together.