Watercolors (US) or Watercolour (UK) (and "aquarelle" in French) is a painting method. A watercolor is the medium or the resulting artwork, in which the paints are made of pigments suspended in a water soluble vehicle. The traditional and most common support for watercolor paintings is paper; other supports include papyrus, bark papers, plastics, vellum or leather, fabric, wood, and canvas. In East Asia, watercolor painting with inks is referred to as brush painting or scroll painting.

Several factors contributed to the diffusion of watercolor painting during the 18th century, particularly in England. Among the elite and aristocratic classes, watercolor painting was one of the incidental adornments of a good education, especially for women. By contrast, watercoloring was also valued by surveyors, mapmakers, military officers and engineers for its usefulness in depicting properties, terrain, fortifications or geology in the field and for illustrating public works or commissioned projects. Watercolor artists were commonly brought with the geological or archaeological expeditions funded by the Society of Dilettanti (founded in 1733) to document discoveries in the Mediterranean, Asia and the New World. These stimulated the demand for topographical painters who churned out memento paintings of famous sites (and sights) along the Grand Tour to Italy that was traveled by every fashionable young man or woman of the time. In the late 18th century, the English cleric William Gilpin wrote a series of hugely popular books describing his "picturesque" journeys throughout rural England and illustrated with his own sentimentalized monochrome watercolors of river valleys, ancient castles and abandoned churches; his example popularized watercolors as a form of personal tourist journal. The confluence of these cultural, engineering, scientific, tourist and amateur interests culminated in the celebration and promotion of watercolor as a distinctly English "national art". Among the many significant watercolor artists of this period were Thomas Gainsborough, John Robert Cozens, Francis Towne, Michelangelo Rooker, William Pars, Thomas Hearne and John Warwick Smith. William Blake published several books of hand tinted engraved poetry, illustrations to Dante's Inferno, and also experimented with large monotype works in watercolor.

Watercolor painting also became popular in the United States during middle 19th century; the American Society of Painters in Watercolor (now the American Watercolor Society) was founded in 1866. Major 19th century American exponents of the medium included William Trost Richards, Fidelia Bridges, Thomas Moran, Thomas Eakins, Henry Roderick Newman, John LaFarge and, preeminently, Winslow Homer. The popularity of watercolors stimulated many innovations, including heavier and more heavily sized wove papers and brushes (called "pencils") manufactured expressly for watercolor painting. Watercolor tutorials were first published in this period by Varley, Cox and others, innovating the step-by-step painting instructions that still characterizes the genre today; "

The Elements of Drawing", a watercolor tutorial by the English art critic John Ruskin, has been out of print only once since it was first published in 1857. Commercial paintmaking brands appeared and paints were packaged in metal tubes or as dry cakes that could be "rubbed out" (dissolved) in studio porcelain or used in portable metal paint boxes in the field. Contemporary breakthroughs in chemistry made many new pigments available, including prussian blue, ultramarine blue, cobalt blue, viridian, cobalt violet, cadmium yellow, aureolin (potassium cobaltinitrite), zinc white and a wide range of carmine and madder lakes. These in turn stimulated a greater use of color throughout all painting media, but in English watercolors particularly by the Pre-Raphaelite painters.

Watercolor was less popular on the Continent, though many fine examples were produced by French painters, including Eugène Delacroix, François Marius Granet, Henri-Joseph Harpignies and the satirist Honore Daumier.

Unfortunately the careless and excessive adoption of brightly colored, petroleum derived aniline dyes (and pigments compounded from them), which all fade rapidly on exposure to light. and the efforts to properly conserve the 20,000 Turner paintings inherited by the British Museum in 1857, led to an examination and negative re-evaluation of the permanence of pigments in watercolor. This caused a sharp decline in their status and market value. Nevertheless, isolated exponents continued to prefer and develop the medium into the 20th century. In Europe, gorgeous landscape and maritime watercolors were produced by Paul Signac, and Paul Cézanne developed a watercolor painting style consisting entirely of overlapping small glazes of pure color.

In America, the most famous watercolor artists from this period include Maurice Prendergast, Frederick Childe Hassam, Charles Webster Hawthorne and John Singer Sargent, considered by many the finest watercolor painter of all time.