A few companies do color argonite sand to be used in the SW aquarium. Make sure that it is argonite sand intended for SW use and then it should be fine. It is usually really expensive so I don't think I'd use it in a big tank. I probably wouldn't ever use it because I prefer a natural look, but if you like the dramatic effect, go for it. Black sand might be fun, and it could even look natural since there ARE black sand beaches.