Carbon Black Hydrus Watercolor by Dr. Ph. Martin’s. Dr. Ph. Martin’s Hydrus Fine Art Watercolor is lightfast, transparent, fine art quality watercolor in a glass bottle. This liquid product delivers the same brilliance and permanency as traditional tube watercolors. Made from the finest artist pigments, Dr. Ph. Martin’s Hydrus Fine Art Watercolor can be blended and intermixed while still maintaining the transparency and luminosity of the colors. Each set comes in a tray that doubles as a mixing palette and bottle holder, with space for a water cup and twelve individual mixing wells. Great for use in technical, calligraphy, fountain pens, and in airbrushes.