Costumes & Props

Lots of horror films use dark coloured clothing throughout the film which often helps set the dark mood of the film and creates a sense of bleakness. Often the clothes that the characters wear are just normal/casual clothes which is the director attempting to make the characters and the film seem more realistic.
Seeing the characters dressed in strange or out of character costumes would have the audience confused, leaving them unable to relate to the characters which is key to making horror films scary.

The audience must feel part of the film or must feel a connection through the things they see like costumes and props. A normal kitchen knife becomes a psychopaths weapon of choice or the victims only defence against pure evil. A necklace you wear or a wrist band you own could be like the charm or cursed one from the horror film, these things are all key to creating the all important element of fear.