Both watercolor and gouache are made from the same basic materials but have important differences.
Watercolors have pigments with very small particle sizes. This allows the paint to be spread thinly enough that it is nearly transparent. Gouache on the other side has more body and larger particles. It dries more opaque, appears heavier and denser and is therefore more opaque. It is best to use the paint to create a flat, matte wash of color. Gouache...