I’m almost angry that I wasn’t turned onto these at some point in school. I’m not even sure where I first saw them, but once you’re after them you will probably find them waiting for you on the shelves of your local art supply shop. You’ll see them hanging there, just looking at you like you’re an idiot for having not picked one up years earlier.
Watercolor is just about the most perfect mobile paint medium except for the part where you have to lug around clean water or be near a source (river/lake) if painting on location. These will change the way you work if you’ve never used them. It makes things easier, but also opens up new possibilities. We use various sizes, favoring the large. We’ve seen some people fill them with pigments, but we worry about damaging the tool. However, we do have one filled with a clear granulation-medium that works well with pigment based tube paints and after years of being filled, hasn’t shown any signs of damage to the internal seals.