In some cases, skintones don’t have to be normal. If you’re subject isn’t really “human,” they don’t need natural skintones.
Normal looking skintones are need for normal people, but what if you’re subject isn’t “normal”? Therein lies and exception to the skintone guideline, one that especially applies to horror movies. Vampires are usually pretty ashen, almost completely white:
One thing that happens in a lot of these movies (but not all) is keeping human skintones normal while vampire skintones stay desaturated (see Twilight example).
Another example is zombies, which are usually greenish or ashen/white:
If your subject isn’t human, it isn’t under the same rules as human, so use that to your advantage to make them stand out, but be sure you know the characteristics of whatever subject you have (if they’ve been done before).
I’ll leave you with one more example :)