That might be the rationale for why they did it. But, if so, it's a dumbass reason. This is STAR WARS for Pete's sake. We kind of expect that there's going to be new weird stuff we have to figure out. There hasn't been a precedent for explaining every odd thing to the point we need to figure it out immediately. It's not like Rey had to to whisper to Kylo, "The Praetorian Guards have a special face shield that looks opaque, but they can see out of," to let us know they aren't running around blindly. We just see it and figure, "Well, there's something cool. Maybe I'll google it after the film." If they had this vision of white on top of red, and they wanted us to know that the top layer was salt, then there are a bunch of ways to work that into a conversation that is not licking the ground of a planet you've never been on. I mean, I wouldn't scoop up something on the ground here and taste it, and I know already it's not acid or poison or something. But on another planet? That just seems like a really bad idea. Plus, I was less curious about the salt, and more the stuff under it, since it looked like blood. If they wanted to get out of the way what was the weird contrast of materials, that would have been my question. Anyway, if your hypothesis is correct - and all signs point to it being so - then it's just a clunky way of getting that out in the open for us.