The photographer understood the theatrical nature of official royal photographs, and in fact used a theatrical set designer to create the background. It's also likely this gown is likewise a prop. Diana was never seen wearing it publicly, and the photo session included images of her wearing the same gown but in ivory (below). It's believed the gowns were not actually Diana's, but provided specifically for the photographs, probably through Vogue magazine who had in the past scouted out clothing for Snowden portrait sessions.