Technically, watercolor is the most difficult medium, especially for representationally inclined artists. The surface is special papers that have to be carefully prepared, drying time is very rapid, color mixing is highly complex, over-painting and repainting cannot be done, the color white is not a palette option (white is represented as negative space), materials, especially brushes and paper, are expensive and permanency is poor. Conversely, watercolors provide a luminance and transparency that is unsurpassed and well-worth the technical effort. With sufficient mastery, the artist can create highly representational paintings (e.g., the watercolors of Thomas Eakins and Winslow Homer). Dr. Brown's watercolors are representational.