From the raw it's "はいいろ", haiiro, but I think there are 2 ways to read it:
- as a single word, gray / 灰色
- as "a hero" in english, which would make sense with ch11's mention of a sketchbook full of edgy things and something he'd like to bury.
Like, the persona he used during his shameful chuunibyou period, when he called himself "a hero" because it was cool to keep his identity secret, but nobody understood english and the closest sounding word was 灰色 / haiiro / gray