In the visual arts world, there is a lot of confusion about halftones.
Halftones are found wherever images are illuminated. Shadow tones are everywhere else. We can grasp the concept more easily if we look at what happens to a sphere in direct light.
It is within these magical areas of light where we find nuances--subtle changes in light values and in color temperature--that can give real depth to our work. Master painters like John Singer Sargent had a eye for translating these, a skill that enabled him to do this portrait of Mrs. Henry White.
They seem insignificant, but even the tiniest change in value and/or temperature can make a big difference. And we can learn to see these by closely looking for them.
Once we find them, we can paint them. And once we learn to do that, we can find ways to exaggerate and manipulate them in all kind of creative ways. But seeing comes first. Once we see it, we never forget it.