Funny that you should post this right now... in the past couple hours I've been thinking a lot about the salt used for salt prints. The one I just made today was on paper floated on a solution of kosher salt ( with no additives, not even sodium ferrocyanide ) and 0.5% citric acid. I've also made them from Morton's salt which contains calcium silicate, dextrose and potassium iodide; 3 kinds of sea salt, one of which had calcium silicate added as an anticaking agent, a kosher salt that had sodium ferrocyanide added, and saltwater from a salt marsh.
What is interesting is that they all produce slightly different colors. And they all change color a little differently in the first wash and then in the hypo. The one I made today is a sort of slightly yellowish dark brown, which will get richer when it dries.
I still haven't tried seawater from the beach here, or some neat orange alaea salt I brought back from Hawaii. I find it interesting that they are all different, even different brands of kosher salt. If it's pure NaCl, why is that happening? I don't really want to say this, because it would be sort of fun if the best one was some obscure or rare kind, but so far my favorite color is from the first one I tried that happened to be in our cupboard: the Mortons.