I’ve always thoughts dimness was more frightening than darkness. 

I’ve had dreams throughout my life where the only light is a dim, dusty gray. I was always frightened in these dreams. Objects were just on the edge of visible, and took on a strange character, as if the gray light changed their natures into something foul, perhaps evil. 

I get why darkness is scary, obviously: we can’t see. But it does trigger our other sense, enhancing our hearing, smell and even touch. Certainly, if I was in a dark forest and something growled, I’d be freaking out. 

But in the forest at dusk, every little tree creak and stick break snaps our attention to indistinct shapes. In the dim, the hump of a boulder could be the body of a bear. A tangle of branches could be the limbs of a monster. A soft sound on leaves and pine needles might be the stealthy padding of hunting wolves. 

That is why in my stories, the color of evil is not black, but gray.