Shiritsu!! Kurokami Gakuen

It was a school filled with nothing but black hair. Red, blue, yellow, green— Modern society is overflowing with a multitude of colors. For the protagonist, Himari Tamura, it is the norm to see people with all sorts of hair colors living freely. However, due to her parents' work, she transfers to [Private Genjo Academy], a school renowned for its strict emphasis on discipline and decorum. As she steps through the gates, filled with a mix of anticipation and anxiety, she is suddenly stopped by a shout: "Hey! What is that hair color?!" She looks around, only to find nothing but students with black hair... (Official Page Site - Animate games MTL)