Who knew?

On black and white film, hues of red become dark shades of black. Pink is the perfect balance to give it that dark creepy grey.

A related fun fact: while old black and white film was under-sensitive to reds, it was correspondingly over-sensitive to greens. Actors whose characters were meant to have unnaturally pale complexions - like Morticia Addams - would often take advantage of this by wearing makeup with a green base tint in order to make their faces “pop”. This is where the modern trope of cartoon vampires having green skin comes from.