Sue Oujo no Koshi Ire: Sono Kage de Hamerare, Tsukai Suterareta Oujo no Kagemusha no Shoujo ga Jibun no Shiawase wo Tsukamu Made

Mary is an orphan who has been blessed with the special ability to copy other people's hair color. As a body double for Leonora, the youngest princess of the Granberg Kingdom, she is subjected to a strict lady's education, and before she knows it, Leonora is dumping most of her work on her. One day, the eldest son of the Marquis, Alberto, is chosen as Leonora's potential fiancé. However, Leonora is dissatisfied with the engagement and orders Mary to find some flaws in Alberto in order to break it off...!? (Source: Kodansha, translated)