pigment ground in gum, usually gum arabic, and applied with brush and water to a painting surface, usually paper; the term also denotes a work of art executed in this medium. The pigment is ordinarily transparent but can be made opaque by mixing with a whiting and in this form is known as body colour, or gouache (q.v.); it can also be mixed with casein, a phosphoprotein of milk.

Watercolour compares in range and variety with any other painting method. Transparent watercolour allows for a freshness and luminosity in its washes and for a deft calligraphic brushwork that makes it a most alluring medium. There is one basic difference between transparent watercolour and all other heavy painting mediums—its transparency. The oil painter can paint one opaque colour over another until he has achieved his desired result. The whites are created with opaque white. The watercolourist's approach is the opposite. In essence, instead of building up he leaves out. The white paper creates the whites. The darkest accents may be placed on the paper with the pigment as it comes out of the tube or with very little water mixed with it. Otherwise the colours are diluted with water. The more water in the wash, the more the paper affects the colours; for example, vermilion, a warm red, will gradually turn into a cool pink as it is thinned with more water.

The dry-brush technique—the use of the brush containing pigment but little water, dragged over the rough surface of the paper—creates various granular effects similar to those of crayon drawing. Whole compositions can be made in this way. This technique also may be used over dull washes to enliven them.