Values make the portrait, not the colour.
Values in portrait painting refer to the relation of lights and darks. This contrast is more important than the colour contrast. As is is actually in every painting.
Hence, in portrait painting it is really fundamental to pay attention to this mutual relation.
It is a challenge to learn to see and use the value contrasts. Ask yourself always which tone is lighter or darker than the surrounding tonal values.
One of the easy-to-use tools is squint!
By almost closing your eyes a large part of the colour disappears and the light-dark contrast remains to be judged.
It is the cornerstone to paint a good portrait. Consequently, the importance of good shadows is consistent. If there is hardly any shadow on the face, like in frontal lighting, the resemblance must be made out of subtle colour changes. That is more difficult than with distinct shadows.
See also : What is Chiaroscuro?