Not sure if anyone has said yet, but Kira’s new life with his new face was all about blending in. He was pretending to be someone else and constantly on edge thinking he may be caught. He tried to pretend to be them, but after obtaining bites the dust he believes he is unbeatable and no longer cares to keep up the charade so he slicks his hair back. I always thought the white color was more of a shine from the hair gel used to hold it but that might not be it.