Whatever kind best stimulates our own individual imaginations to evoke the horror that most scares us. For example, the original movie The Thing in black and white.
The longer the teases and hints fuel our imaginations, the scarier everything gets. The instant any monster is revealed we start to cope with it, usually looking for a way to laugh at it and mitigate our fear so we can start thinking about specific defenses against it.
Incidentally, note the paradoxical effect of color in monster movies. For example, no matter what slime or goo connotations are intended, green remains mostly a friendly color.
The less light, and hence less color, the more we must use our imaginations on the beast and its threat rather than color cues that often diminish rather than increase the fear.
The original b&w version of The Thing was much scarier than its sequel in color. Same for many of the Frankenstein, werewolf, and vampire movies.
Oh yeah, one more key ingredient: awful terrible demented SOUNDS! Especially off camera. A deep rumbling growl from the bowels of hell. Slow labored raspy gasps for breath. A sudden shriek that loosens bowels. A howling that soaks us in sweat. Ripping followed by gurgling. Sickening thuds. And we never saw a thing, which only makes it worse.
