Gouache is a little known media for creating paintings. It's most similar to watercolor paint, although it can produce results that are similar to oils or acrylics. It has actually been around for quite a while. Some speculate that it has been around for about 800 years. Gouache has been called opaque watercolor. When used in thick applications it is opaque, however much like acrylic, it can be thinned to produce marks that are just like watercolor. Gouache is relatively inexpensive compared to oils and acrylics, making it a great choice for beginners. Because gouache dries very quickly, it is great for sketching on site. Gouache has been a medium used by illustrators because of its quick drying time.
Gouache can be used on a variety of surfaces. Because it is water-based, it is a good idea to pick a surface that will accept water. Watercolor paper is an excellent choice, however illustration board or gessoed masonite will also produce excellent results. A variety of brushes can be used with gouache. I usually use nylon brushes, although bristle brushes will create a spectrum of textures.
On thing to keep in mind with gouache is its permanence. Gouache is reactivated when exposed to water. This is both a positive and a negative. Finished gouache paintings may need to be protected to keep moisture away. However, retouching and lifting of colors can be achieved on paintings that have dried. Gouache can also be reactivated on a palette as well, allowing an artist to keep paints for a long period of time.
One major difference with gouache is the use of white. Most watercolor artists rely on the white of the paper to produce lighter values. With gouache, this is not the case. Artists mix colors much like with acrylics and oils, using white to mix tints. This means that gouache can used on toned paper, unlike watercolor.