Watercolor compares in range and variety with any other painting method. Transparent watercolor 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 watercolor and all other heavy painting mediums--its transparency.
The oil painter can paint one opaque color over another until he has achieved his desired result. The whites are created with opaque white. The watercolorist'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 colors are diluted with water. The more water in the wash, the more the paper affects the colors; for example, vermilion, a warm red, will gradually turn into a cool pink as it is thinned with more water.
An unpredictable medium, the character of watercolor is uniquely challenging. The accomplished watercolorist learns to take advantage of the unexpected results of the medium. As practiced by most of its greatest masters, spontaneity is everything. The artist learns to improvise, which can be done effectively only with experience. The intimacy of the medium springs from the way it encourages improvisation and seems to record the artist's fleeting thought on paper.
People have been told that watercolors will fade. This was the case of older watercolors. Many of the famous artists such as the British Watercolorist used materials and paints that were fragile and not archival. This is not the case of today's artists. The paints have been reformulated and the longevity is unsurpassed. The surface that was once used to paint on was not archival and would pass the test of time. Today's watercolorists use 100% rag paper made with cotton and guaranteed to be acid free and archival. So you see today's collectors have no need to worry.
Excerpted from the Encyclopedia Britannica Online and Springfield Art Museum