Apparently, it's because the color inside a mother's womb is pink. It's the color of the world she was completely enveloped in for 10 months before being born. That's why everyone seems to like pink. However, Momo's mom was unwell, so the color of the mucous membrane in her womb might have been a bit off.
Because ever since Momo was old enough to understand, purple has always been her favorite color.