They use a RAM 336 for printing it seems that the color modulation takes place in the curing process so heat is involved. In my experience there is no way to switch between different sands in process.
I had the same thought. The answer is actually in the text, took a while to find it:
"Forust’s 3D printed parts support a wide variety of wood grains at launch, including rosewood, ash, zebrano, ebony, and mahogany. The parts also support a range of wood stains such as natural, oak, ash, and walnut."
Yeah it seems it's just painted. So now you can't even trust wood anymore. Very tacky.