Why Use a Yellow Underpainting?

Painting in gouache over a yellow casein layer puts “fire in the belly“ of a gray painting. From my Gumroad tutorial Triads: Painting with Three Colors: CASEIN UNDERPAINTING Richeson casein: GOUACHE TOP LAYERS: Yellow ochre: Prussian blue: Light red: Titanium white: MATERIALS: Watercolor sketchbook: Travel brush set: Water cup: Homemade easel: Tripod: Water-soluble colored pencils set: