That's a great idea. Do you have any other ideas for finishing the other half of the pieces. Graphite seems too messy, and I'm not sure the different sands will look different enough.

I imagine the details that currently (and insufficiently in my opinion) distinguish the colors would be obliterated.

In the spirit of Dune, you should dip them in spice melange. However, given current shortages, you could get away with using iron oxide instead.

There’s black sand! Volcanic sand from Iceland is perfectly black and would be a great way to distinguish them

You can also by any color of "sand" you like at your local craft/hobby shop.

Glue on silica carbide. It's usually darker colored and has a very aggressive texture (used for grips on pistol handles and skateboards among other things).

Fuzzy skin finish might also get you pretty close along with a wood PLA. They can do both wall and top layer fuzzy skin now.

You don't need wood PLA, I have a Bambu matte ecru PLA and with fuzzy skin it looks almost exactly like this.