Generally speaking [for a strong painting], you really only need three or four values. Sometimes you need five, depending on the environment. But that is enough. And then… you let hue do the rest of the work.Jill Carver on The Plein Air Podcast
Not that this sounds easy to pull off!
She also recommended doing notans, which I’m sure would help a lot with the first part of this equation. I think I’m going to buy some brush pens and do some of those.