Q. How does gouache compare to transparent watercolor, and can they be used together?
A. Traditional gouache and transparent watercolor are, in fact, the same paint “species.” They’re both formulated with fine gum as the binder (typically gum kordofan, a specialized variety of gum arabic from acacia trees in Africa). And both become bound to the painting surface through a marriage between the gum and the absorptive surface of the paper.
The key difference is in their relative opacity and flow. There are two ways for the manufacturer to make gouache opaque. The first is through the addition of opacifiers (which inevitably compromise and dull the true nature of the pigment). The other is through color load, which entails adding enough pigment to ensure the complete opacity of the paint film. Certainly that route is better, ensuring opacity while not sacrificing any of the unique properties that come with each pigment.
Gouache can easily be used in concert with transparent watercolor, with a couple of caveats. First, gouache doesn’t dilute with water the same way traditional transparent watercolor does; you won’t get the same flowing washes with the clarity and brilliance possible with a fine transparent watercolor. But you will get a level, dense film of intense color.
Second, because gouache was originally used in the design community, only recently has there been demand for colors that offer the same degree of lightfastness and permanence as required by fine artists using transparent watercolor. Many lines of artists’ quality gouache offer a number of permanent colors, but a significant number are less than fully lightfast. So it’s important to check the lightfastness or permanence rating on the tube.
Typically, gouache is used for adding bright, crisp highlights over transparent watercolor (for example, it’s unbeatable for adding spray to an ocean wave). It can also be used to add a counterpoint of dense and saturated color in a way that transparent watercolor simply can’t match.