Change the poster.

I didn't like the poster either... but the director is a particular kind of film freak, and the poster/cover art has to be to him, something that would've got his attention in the eighties.

One of the reasons for the film was when he was young his parents wouldn't allow him to see r-rated violent or scary films. He'd go to the video store and absorb everything from the movie boxes, deciding himself how the film would work itself out. Specific story elements and how they'd play out, and of course when he later saw the films, they were nothing like he'd imagined.

He said he wanted this film to feel a little like one of those created movies.

I am not very keen on it either. As a kid who grew up (or something) watching freaky horror and sci-fi tapes in the 1980s, I didn't even like most of the box art then. Much of it tried to overcompensate for the low-budget locations and effects of crudely made films, depicting the monsters or action as far more exciting than they turned out to be. Some of the box art was adapted from the original posters and other promotional materials, and even then I often thought that they made poor selections. They always chose the art which seemed most lurid, even if it was far from the best rendered or most creative.

They poster for Beyond the Black Rainbow isn't terrible, I guess... it shows the main characters, the control pyramid, Nyles' grasping, destructive need to possess Elena. And nothing else! So, it certainly is focused, if nothing else. But I think it's awful at conveying the tone of the movie, its visual character.

I am not much of a graphic designer, but if I was considering elements for the poster, I'd try to include: color, symmetry, patterns, expressionist lighting and scene framing, and a human element which conveys the terror of biding one's time while being stalked. Something starkly primal amidst the formal, structured environment.