And looking for my chilhood book...

... you could see the basic difference from watercolour and gouache, the watercolour different overlay other colours the result are always add and the white is you be careful and don't paint the paper, so the white is the colour of the paper and the gouache is could you paint an overlay if you add a stroke in the painting the colour would not ad to the others and you could make the light and white painting over the other paint colour part, the mixture of colours is made in a plate and not added to the paper. Rute Bezerra de Menezes Gondim