(1.) Gouache is Thicker
Gouache has a higher ratio of pigment to binder than watercolor does. The white of the paper or other substrate will not show through the paint and change the appearance of the finished color like transparent watercolors will. Water is added to gouache to lower the viscosity and make application smoother, much like watercolor.
(2.) You Can’t Water it Down to Go Lighter
Gouache colors must be lightened by adding white gouache or pigment to lighten the hue, sort of like oil paints. Diluting gouache with water does not lighten it and make translucent in the way that regular watercolors can be lightened.
(3.) It’s Not for Tinting and Glazing
Gouache does not need to be applied in multiple layers to build up colors like with watercolor. Gouache is very opaque and flat. Watercolor can easily granulate, blossom in uneven washes and take on the textures of toothy papers. Gouache dries very evenly and cracks when applied too thickly, so most textures must be implied.
(4.) Gouache is Flawless
Well, maybe not totally flawless. If it’s applied too thickly, the paint can crack while drying. Gouache does have a flawless color laydown. Colors look dense and true and even when they dry. It does not pool or blossom in the uneven manner as watercolor does. This is part of the reason it was so popular in poster design, because gouache designs can be easily photographed and reproduced. They look crisper than watercolor images and the colors are very strong.
(5.) You Can Go Light Over Dark
In most painting techniques, and especially in watercolor, artists will lay darker colors on top of lighter colors. This is not necessary with gouache because it is so opaque. Gouache covers all paint layers, so even a dark background can be painted first and lighter colors can be layered on top of it without altering color brightness.
Check out our other post on gouache for more detailed information about this awesome medium!