When the first explorers returned with the hides of the misnomered Flying Monkey, researchers back home guessed that the colors made for a dazzling mating display. It was only after the xenophotographer Ly Bishop lugged his wide spectrum equipment into the jungles that they realized that the creature's skin let it blend in with the colorful chaos of the foliage.
I actually rather like this design for "Sand Hunters":
That's a MUCH more convincing concept for an Arakeen still-suit than any I've ever seen. Leaving the face bear in a desert climate as deadly dry as Dune would prettymuch leave your most vulnerable moist membranes—your naso-gastric region and your eyes—bare to the elements. And the constant windblown sand would prettymuch erode your face away in a few days if you're out in the elements for more than...maybe three minutes every few hours.