Acrylcolour: Why don’t the colours need Acrylcolour protective varnish to be protected?
They do not need a protective coating to the extent that it is not but gouache acrylic paint. You may, however, if you wish, apply a varnish.
Acrylcolour: What is the difference between an acrylic and gouache?
Once dry, gouache is re-soluble, but acrylic is indelible.