Tom Felton: A Tour of Diagon Alley
Look at the sky. It’s not dark and black and without character. The black is, in fact deep blue. And over there: lighter blue and blowing through the blues and blackness the winds swirling through the air and then shining, burning, bursting through: the stars!
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.
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.
These are some fun fucking facts