What moves you to add a slash of green to an area of skin tone?
In painting from life, when do you make a discerning judgement about color and value as you see it, and capture that, vs using color theory and mood to improvise more creative color, while still working in a naturalistic/realistic vein?
because skin is a myriad of different colours. It show mores when painting from real life as opposed to photos.
The best way to do this is to experiment and to meditate.
Try using undercolor for your painting that you would consider unusual for your subject. Maybe you'll draw a portrait on a blue surface, and allow some of the blue to show through your brushstrokes. Maybe you'll try adding a bit of green to your shadow tones. Or maybe you'll add a bit of red to your lights. Feel free to go crazy and try everything you think of. Maybe try to use every primary color in your skin tone, circulating them throughout. Eventually you'll learn what works.
Examine and meditate upon your environment. Look at the trees and study them. Where you once saw only green leaves, you will come to see hints of blue and purple and red in the way the light mingles among the leaves. Where you once saw only peach-toned skin, you'll come to see the redness beneath the skin, and the cool lights cast upon the surface of the skin. Let your meditations inform your experiments.
Never stop doing either of these.