I find grotesque or spooky pictures appealing, too.
I think it is interesting not to rely on clumsy gross-out-effects, but to make the depicted situation in it’s entirety disturbing or unsettling.
This might be interesting for you as well:
On of the roles of the composition of an image is to depict the most important elements of the picture in a clear, recognizable manner. For color pictures, not only the placement of the elements is playing a central role, but the color composition as well.
An obvious method is to separate diffrent elements or layers within the image like foreground and background by different color saturations of a difference in lightness.
There are many approaches for setting up a color composition. To acquire some helpful rules of thumb, it can be useful to look at some of the oldest works of visual design in history: Coat of arms and flags.