@hazhir
Gouache is a water-soluble and opaque paint so the white of the paper surface does not show through.
It is a tad chalkier than traditional watercolor and absorbs light rather than reflects it. The paint is diluted with water, but possesses a strong colored pigment.
This unique paint dries darker and a little of it goes a long way and takes time to dry which means it can be workable for hours or even days after application.