Gouache (pr. gwahsh) is a heavy, opaque watercolor paint that produces a more strongly colored picture than ordinary watercolor.  It can be used thin or thick and is a favorite medium among illustrators due to its quick drying time.  Its use goes back to the Egyptians and, more recently, has been used by such artists as Vincent Van Gogh, Paul Klee, Pablo Picasso and . . . Tibor Gergely.