A: Gouache and watercolor are really similar. The two types of paint have practically the same pigments, binders, and formulation, except for one thing: gouache colors are formulated for opacity, while watercolors are designed for transparency. That’s not to say that some gouache colors aren’t transparent, or that some watercolors aren’t opaque; white and Naples yellow are both commonly available in watercolors, while many dark colors in gouache tend to be quite transparent. The fundamental difference is in the way they are used.
In traditional watercolor, the source of white, and light, in painting is provided by the reflective power of the paper. By using transparent colors over a heavily sized (gelatin-impregnated and coated) paper, artists achieve a sparkling brilliance, as light reflects through glazes of clear color. This technique allows artists to achieve great intensity and depth of color, while using a minimal amount of paint. The texture of watercolor paper, which can be rough and aggressive, enhances this effect, while breaking up surface tension and preventing paint from pooling and running excessively.
Gouache, like oils and acrylics, is mixed with white paint to achieve tints and light values. Gouache is opaque and matte, and re-wets easily. The flat, shine-free surface of gouache makes it a preferred medium for illustrators. Because it readily re-activates with water, gouache layers tend to intermix, and darker colors will show through lighter ones. Some artists use this to their advantage, and plan for lines and details to come through top coats. Other artists like to start with lighter colors first, and work their way toward dark ones so that nothing bleeds through.
Because watercolor and gouache are so similar in composition, the two types of paint can be intermixed. Watercolor can even be used like gouache by adding white paint on the palette, though the resulting paint surface might not be as matte as gouache. Likewise, gouache can be used in watercolor painting, though colors may not be quite as deep and brilliant as watercolor. Leftover gouache and watercolor can both be left on the palette for later use.
Artists who like the consistency, opacity, and matte finish of gouache, but prefer a paint which does not intermix between layers, or which can be framed without glass, should consider trying acrylic gouache. This variation on acrylic paint features the matte appearance, fluid body, and opacity of gouache, in an acrylic formula that is durable and resists re-wetting.