Rikan Koukoku Hikikomori Zuichou Hiden - Hibi Koukyuu no Nukedashi, Yuunou Kanri Yattemasu.

https://www.cmoa.jp/title/2633... Lost in a Chinese-inspired fantasy world where only the emperor and his consort may possess the noble color black, a Japanese woman named Saya is taken in by the emperor of the Rikan Empire and becomes one of his consorts. But when palace life grows unbearably dull, she sneaks out, disguises herself as a man, and begins working as an official—only to find herself quickly climbing the ranks! “Work is fun… but if they find out I have black hair—or worse, that I’m a woman—I’m doomed.” While concealing both her identity and her forbidden black hair, Saya deftly switches between her two personas—the “hidden consort” and the “brilliant official”—to solve the court’s most perilous mysteries and protect the empire from the shadows.