given that tsukasa, saki, and akito’s hair is canonically natural (or at least akito’s), and presuming that ena’s is natural, i figure rui’s hair being natural is honestly not far fetched in the proseka universe (considering sekais exist multi coloured vibrant hair is honestly a minor thing to consider “normal”….) but i am not opposed to the idea of mamashiro and papashiro dying rui’s hair when he was young and now he knows no different and continues to dye it himself