If you are using watercolor in the field, consider this terrific tool–a brush that holds water in the handle so you do not need to dip it into water to paint. It is very handy for quick sketching, especially in the field. Though there are water brushes in many sizes made by many companies, the Pentel Arts® Aquash™ Large (18mm), Fine Point Water Brush is by far my favorite. If you use a waterbrush, carry a rag to wipe the brush clean to change colors. (I use an old sock with the toe cut off and put it around my wrist so my rag is accessible any time.) If you use this brush, you can also dispense with bringing the tightly sealed water bottle for painting. Once you try it, you may never go back to traditional (and more expensive) brushes for your field work. I now use a Pentel Water Brush almost exclusively, even for my studio work.