Using Watercolors

I love wikipedia! http://en.wikipedia.org/wiki/Watercolors

Watercolor (or watercolour, also known as aquarelle) designates either a painting method, the medium, or the resulting artwork, where paints made of pigments suspended or dissolved in water are used. Although the grounds used in watercolor painting vary, the most common is paper. Others include papyrus, bark papers, plastics, vellum or leather, fabric, wood, and canvas.

Paint
The broader term for water-based painting media is "watermedia". The term "watercolor" most often to refers to traditional transparent watercolor or gouache (an opaque form of the same paint), but also includes the use of thinned acrylic paint.

Traditional watercolor paint is made of finely-ground pigment mixed with gum arabic for body, and glycerin or honey for viscosity and to bond the colorant to the painting surface. Unpigmented filler is added to gouache to lend opacity to the paint. Oil of clove is used to prevent mold.

Watercolor paints vary in their transparency, some being less transparent (more covering) than others. The more transparent paints allow the paper (or an undercolor) to show through while others allow less of the undercolor to be seen.

As there is no transparent white watercolor, the white parts of a watercolor painting are most often areas of the paper "reserved" (left unpainted) and allowed to be seen in the finished work. White paint might be used to indicate snow on a fence or the foam in the sea, as examples, by using Chinese White or White Gouache. These are not transparent. Traditionally, such non-transparent paint is used sparingly so as not to lose the light and airy look of the work.

Some watercolor pigments are "fugitive", meaning they fade over time when exposed to light. An example is Alizarin Crimson. Some paint makers offer a different formulation of pigment as a less-fugitive alternative. These often have the word "hue" as part of the name. "Alizarin Crimson Hue" can be expected to be less fugitive than Alizarin Crimson.
"Staining" is another characteristic of certain watercolor pigments. A staining color is difficult to remove and persists on the paper. Less staining colors can be lightened or removed almost entirely when wet or when re-wetted by "lifting" with a wet brush, paper towel, tissue, sponge, or similar.

Commercial watercolor paints come in two grades: "Artist" (or "Professional") and "Student". Artist quality paints are usually formulated using a single pigment, which results in richer color and vibrant mixes. Student grade paints have less pigment, and often are formulated using two or more less expensive pigments. Artist and Professional paints are more expensive but many consider the quality worth the higher cost.
Paint pigments and formulation vary among manufacturers. Paints with the same color name from different makers can vary in hue, staining, and other characteristics.