Brush-Up Paper is a product put out by
Loew Cornell, advertised as "The fast, easy and inexpensive way to practic your brush strokes over and over again."
It's a sheet of specially treated light mottle-gray paper that turns very dark gray when "painted" on with water. The very useful thing about this is that it dries up within minutes (or sometimes even seconds), so you can reuse it. It doesn't get soggy or warped like normal paper and the color change with plain water is as drastic as if it were paint. You can practice on it if you're working on a painting and need just a certain texture but are afraid of messing it up on your work, you can do it over and over, without damaging your art or wasting lots of blank paper and paint practicing the 'old fashioned' way. Its two drawbacks are that water is thinner than most paint, so your brushstrokes end up runnier than they would in the final, and that if you're painting fine detail, the first parts will have dried and vanished before you finish the rest. But other than that, it's a fairly useful tool.