I think there just comes a point where you just have to err on the side of making sure the movie looks good too. And the fremen clotting is one of the biggest things that would dork that up.
Form fitting brown body suits with no features on them. With full head/hair coverings, and face coverings. With brown cloaks covering up most of that. In a brown setting. Regardless of the accuracy or realism that would come from that, people, in general, would hate it. The mind of the viewer would wander, interest would drop, and they'd come away having been bored by any scene with them in it.