Hmm. I haven't tried it but Ive used white sand, and have some black sand, I can mix some today and post some pics if you want, although thats not a promise as the black sand doesnt belong to me, but i can check.
Im guessing it would kind of look like poppy seed, or an overall grey. otherwise a gimmicky kind of dalmation color.
It would be cool if it all became a mixed color, fish crap and stuff would show up a lot less.