Watercolor's defining characteristic — that the white of the paper is the lightest value in the painting, because the medium is transparent rather than opaque — determines everything about how it looks and what it can do. Unlike oil or acrylic, you cannot paint light over dark in traditional watercolor. The light areas must be planned and preserved from the beginning. The medium rewards planning and punishes revision.\n\nThe practical appeal was portability. Oil painting requires elaborate equipment; watercolor required a small box of pans, a brush, and a pad of paper. British landscape painters of the late 18th and early 19th centuries — J.M.W. Turner and John Constable most prominently — developed watercolor into a serious fine art medium precisely because it allowed them to paint outdoors, directly observing the light and atmosphere they wanted to capture. Turner's watercolor sketches of Venice, made in the 1840s, are among the most technically sophisticated paintings ever made in any medium.\n\nThe impressionist transparency of the medium — the way colors mix optically on the wet surface, the soft blooms where wet paint meets wet paper, the hard edges of dried washes — became an aesthetic in its own right, signaling a specific relationship between artist and subject: observational, sensitive, time-aware.