This idea also applies when painting the mid-value and light areas. While the dark areas are still partially wet, I paint the mid-value areas. In most cases, pure colors are laid in side by side on the wet paper, allowing water to mix the colors naturally by diffusion. I'll often add new pigment or simply drag pigment from the existing dark areas. The trick is to maintain just the right amount of water in the brush.
More water creates exciting runs, while less water allows more control.
As I experiment with water and color transitions, I remind myself to hold to the mid-value range. Also, I try to save plenty of white paper for my lights, because at this stage it's easy to cover too much white paper. The painting is now nearly complete. The light areas sparkle with the brilliance of sunlight. But often, some lights will appear too cold and some too white.