Just because the oldsters remarked on her dark skin does not mean that Roland cannot be black - maybe just not as dark (??)Not really...if you recall, when the Ka-tet arrives in River Crossing, the oldsters all remark over Susannah's dark skin, like the people of Garlan had. (Interesting tie-in with the Garlan in "Eyes".) So Roland can't be black, or he would also have been described that way.
But that's a tiny detail - I do get your meaning and would probably accept a black Roland on screen. Black Red didn't bother me nearly like the other changes in Shawshank did. (altered timeline, disposition of some characters, etc.)