Within the Imperial City complex in Huế, Vietnam, is a secondary citadel called in English the "Purple Forbidden City". (Or, per some sources, the "Forbidden Purple City.")
According to my guide book1:
The Forbidden Purple City, enclosed by a low wall, was the personal domain of the emperor; only eunuch servants and concubines were allowed in.
The Wikipedia page for "Imperial City, Huế" says:
Within the Imperial City is the Purple Forbidden City (Tử cấm thành), a term similar to the Forbidden City in Beijing. Access to the innermost enclosure is restricted to the Nguyễn imperial family.
I can't seem to find why it's called so. What does the word "purple" in the name refer to?
1: "The Rough Guide to Southeast Asia on a Budget," 4th edition, September 2014. ISBN 978-1-40934-559-6. p. 869.