The reason why the room was pink was because 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.
Note that this isn’t necessarily the case in modern black-and-white film. Early black-and-white film was over-sensitive to cold hues and almost wholly insensitive to warm ones, forcing reds in particular to be heavily desaturated in order to show up correctly - as seen in this pink living room. Modern black and white film doesn’t necessarily have this quirk.
On the same note, characters who were meant to appear unnaturally pale - like Morticia Addams - often wore blue or green makeup. The aforementioned over-sensitivity of early black-and-white film stock to cold hues, combined with the blue-green tinge of the mercury-vapour lamps that were common used as stage lighting in the early days of film and television, caused this type of makeup to appear luminously pale on film. It’s from this that the modern practice of depicting cartoon vampires with bright blue or green skin arises.