The ZIG Watercolor System Brush H2O range is perfect for the artist on the go, paint anywhere at any time. All you need to do is fill the barrel with water and away you go! The ZIG Watercolor System Brush H2O is great for any artist on location as this is the cleanest and easiest way to paint. ZIG Watercolor System Brush H2O pens have a retractable valve in them so you have complete control of how much water you release by gently squeezing the barrel.