Gouache Painting Medium Definition. Gouache is a painting medium similar to watercolor, but heavier and more opaque because a gum substance is added to the mixture of ground pigment and water. Gouaches, like watercolors, are usually on paper.
Watercolor is more about light than is gouache.
And, frankly, gouache is a lot easier to control than watercolor. It's a quick, direct medium and works well for a sketchy kind of painting. To a fledgling collector, this practice can be maddening.