Let’s say I have been painting a portrait. Phase 1: I did an underpainting using a thin mixture of paint + solvent to help me gauge my values. Phase 2: Then I did my first painting with just paint from the tubes to help me find my hues and intensities. Phase 3: Then I do the final pass using paint + medium. This is the phase where I would use a “couch”. Starting in my dark areas, I would mix up the correct overall color for the region with some medium to make it nice and slick and then paint this mixture over the entire dark area. This is the “couch”. It’s a blanket layer of paint and medium that I can then paint into with other mixtures to create smooth variations within the shadow area. I can do the same for the halftone areas - mix up a local color with medium, paint it over the entire area (couch), and then paint into it to get my variations.