The wiki provides no sources on why it says its blonde. As you've already stated, it's never mentioned in the series. So where did the colour come from?
I'd say go for the pastel dingy green that Toto has. Sure she's not his biological sister but he did adopt her to be his sister. He must have seen something in her that would confirm his own twisted reality that Mitsuzaki is his sister, a replacement for the real one that died. Even something as simple as happening to have the same hair colour could do it for him.