The rumor about the pink nightgown likely stems from the tradition of dressing deceased women in pink, especially blondes. This tradition is largely symbolic, representing femininity and innocence. However, there's no evidence to suggest that this was actually the case with Marilyn Monroe.
Her funeral was private and her burial outfit was not revealed publicly. The white dress is a detail that has emerged over time, and while not officially confirmed, it is generally accepted as the truth.