Typically, when I’m painting, I’m working on one particular passage or section. The colors in that passage are often one hue (or a small range of hues) and one chroma (or small range of chromas). So in any given passage, what tends most to vary from one spot to another is the value.
Usually, what I do when painting in oil is work with two brushes—one for the lighter sections of that passage and one for the darker parts. The value may have a wide range or a narrow range, but either way it’s helpful to have one brush for each purpose. That’s useful, I think, for two reasons. One is the simple technical point that it’s easy to keep track of two brushes, loading, applying, wiping, mixing, and re-loading. It’s a lot quicker to change the paint on a brush from a dark to a mid tone than it is to change from a dark to a light.
The other way that working with a dark brush and a light brush is in terms of thinking about light. As I’m working on a passage I can think about how the light is affecting it at a particular point. If the primary thing happening is that the section I’m working on is turning toward the light, then I automatically grab the light brush. If it’s turning away, then I grab the dark brush. I find it useful to think in that binary way because that’s how light works: a given point is either toward the light or away from it compared to nearby parts of that passage. By working with two brushes, I always keep that in mind.