I read this manga a few years ago.
It's set in the modern day and it's about a high school student who meets a girl with a sword who wears bunny ears and claims to be his fiancée. She says that he is a prince of a country of magic and he is a candidate for the throne and that she is here to protect him from his siblings, and is then attacked by a girl who can control plants and she is cursed to not be able to sleep. Then, he unlocks the power of the royal family that can nullify all magic through this metal golem ghost thing that floats behind him like a stand from JoJo. The golem could turn into this shoggoth like thing duing either chapter 8 or 9 I think.
Also, it turns out some of the characters are based off of fairy tale characters like the thorn girl is sleeping beauty and the next antagonist I remember was Snow White who has magic that can control up to 7 things using her "dwarves".
As for coloring, it was color for the first few pages like some manga but black and white for the rest. The girl wore a kimono and had a katana.
I believe the title has to do with the word "rabbit" or "bunny" so I tried "kaguya inaba usagi" and such things like that.