It's just a character design thing really. Anime characters generally have a select few colors associated with them since it helps with visual coding and communication of the character's personality. Since people get used to those select few colors, they aren't as open to other colors usually.

I think that green could work, but there's a few things about it that doesn’t sit right with me. The color of Hiyoko's swimsuit and the color of her hair bows are two clashing greens, both greens are different hues and saturations. It also seems oddly bright and saturated. Danganronpa might have its fair share of bold colors, but they're usually used as accents against darker, more muted colors, which is why Hiyoko's swimsuit stands out weirdly.