Gouache is a close cousin of watercolour. It’s made of virtually the same ingredients, except that it’s opaque and matte with added barium sulphate or lime precipitate, and contains glycerine to make it more soluble.
Can watercolour medium be used to modify the properties of gouache?
Watercolour mediums can be used to alter the properties of gouache, but it’s important to use high-quality gouache in order to get your desired results; lower-quality gouache will have less pigment, and won’t be as flexible or transparent.
Some watercolour artists use white gouache instead of watercolour when they need a completely opaque white to mask colours or create dimension.