This is most useful for people who do watercolor painting outside, as you don't have to bring a your jars of water with you. I just like the idea of the waterbrush because I'm lazy.
I was at the art store today and looked for a waterbrush, but they were out. Later I happened to be in Target with my sister and they had some in the kids' art section, 5 for $4. (This is cheaper than the $6ish + shipping I'd have to spend to get one online. I'm also cheap.)
The problem: they were already filled with color!
SPECIAL NOTE: This Instructable is mostly to say "hey, look, cheap alternative" and show pictures of the internal doohickeys of this kind of waterbrush. Here are the steps: 1) take apart; 2) dump out the color and rinse; 3) fill with water. Not intensely challenging.
Step 1: Buy, take home
Luckily, I noted that there was some kind of spacer in place between the color (which looked like food coloring) and the brush tip. Which meant that the tips were still untouched by the color. A sinister plot was hatched.
(There were also other brands, but these were the cheapest and I didn't know at the time if what I planned was possible.)
These are pretty decent, actually. The waterbrushes are a good size, and you don't have to squeeze superhard to get water flowing. They also don't constantly drool water.
I got 'em home and took 'em apart over the sink.