Make it a powerful memory, the happiest you can remember. Allow it to fill you up. A full-bodied patronus is the most difficult to produce but shield forms can also be equally useful against a variety of opponents. Just remember, your patronus can only protect you as long as you stay focused. Think of the happiest thing you can.
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.