I love how the gouache pulls the pigment from the darker watercolors. Because of how the brush is loaded some of the gouache is applied quite thickly & creates a temporary textural relief on the surface, while some is barely visible. This difference is really enjoyable & particularly evident when you rotate the painting thru the light.
Winsor & Newton Payne's Grey Underpainting
Holbein Primary White Gouache Prints with Princeton Neptune Oval Wash Brush
Arches Hot Press Watercolor Block