See the little dappled bits in the background? Salt does that. Put it directly on the paper while the paint's still wet, and it sucks up the water, and thus some of the paint, around it. Can be used in clumps for texture, or sparsely for a snowy look. Fun stuff. ^^
It'll work with any water-soluble medium thin enough for the water to be soaked up. Generally, that means watercolors, but thinned-down acrylics and the like should also work.
salt?
May I ask why? (Doesn't do w/colours)