Working with only black or white I refine the light and dark values on a layer set to “hard light”. This means that everything that is darker than 50% gray is multiplied while everything lighter than 50% gray is screened on the layers beneath. Switching to color with an airbrush, I fill in the color values on a layer beneath my hard-light layer.
The values that were painted in grayscale interact with the color to give the final result.
Here is a time lapse video!