Watercolor is a water-soluble painting medium traditionally applied to paper in thin layers with a soft brush. The principal components of the paint are a gum binder (gum arabic or gum acacia) and paint pigment for color. Watercolor paints are made in both transparent and opaque forms, but when not specified, the term 'watercolor' generally refers to the transparent form. (The opaque form, identical in composition except with added pigment or chalk, is called gouache.) High quality watercolor paints are semi-liquid and packaged in tubes, while student and hobbyist grades usually appear in sets of dry pans of color. Either form requires the artist to add water to bring the paint to a usable consistency.
The distinguishing features of watercolor are its transparent binder and finely ground paint pigments. Together, these characteristics allow the paper support to show through the water-diluted paint, becoming the source of whiteness and luminosity in the painting. Darker areas of a composition can be built up in thin washes instead of a single, dark layer of pigment.
It is a demanding medium, in that while it is easy enough to make mistakes with every medium, it is far harder to correct errors in watercolor. Errors in oil may be painted over or sometimes removed with a solvent, charcoal and pastel can be erased, but there are no erasers for watercolor.
Watercolor paintings often begin with a pencil or charcoal sketch, which shows dark areas, from which an outline sketch is derived and drawn on the watercolor paper. Lines of that scketch may or may not be erased from the final drawing. Watercolor may be combined with physically compatible opaque media such as pastel.
The paper may be prewetted to allow delicate yet controlled spreading of the pigment. A brush with wet pigment may be used to create a light wash on wet paper or a brush with concentrated slightly wetted "dry" pignment gives a heavier effect. "Dry brush on dry paper" gives the darkest results but is most likely to smear. To control smearing, small areas may be rewetted as the paer dries.
Planning the work
Since the white of the paper is an element of the painting, in general, the lightest washes and colors are applied first. While experts can simply avoid them when applying darker colors or more coats, water-resistant removable frisket can be applied to protect them.