That is an interesting point, but I don't think it would take much rewriting. Detta Walker doesn't fear Roland because he's white, she fears him because he's self-evidently deadly and implacable, because of what his training and experiences have made him. Despite being very sharp herself, it's clear to Detta that she can't put anything past Roland under ordinary circumstances. Even people who aren't crazy are freaked out by Roland. (The fact that he was also inside her head for a time greatly disturbed her.) Detta might hate him because of his whiteness, but I think it's accurate to say that Detta has a generalized hatred and contempt for everyone around her. She's the Hyde half of what ultimately forms Susannah's personality. If Roland wasn't white, she would find or invent some other quality to hold against him. I don't think this would be difficult to rewrite, and a screenplay tends to have much less dialogue than a novel. :)