NOTAN is King
Notan is defined as the balance between dark, mid-tone and light values in art. Why is notan important? It is the basic building blocks of any painting before any color is applied and there must be at minimum 3 values (dark, mid-tone and light). Without the strong contrast between the darkest darks and the lightest lights the painting looks flat and uninteresting. Ideally one value should be dominant but the other two not evenly distributed.
If you are working from a reference photo it helps to turn the image into black and white to see the value contrast. I have started taking black and white images of my paintings as they progress to see if I have maintained a good value balance.
Here are three examples of strong NOTAN balance. When a painting looks fantastic as a black and white you know you nailed it!