Okay, I think we've got a rainbow order theme going on here, but I'm not sure because three characters only tenuously fit.

1. Yukari = Violet (her name)
2. Ran = Indigo (her name)
3. Kogasa = Blue (??)
4. Parsee = Green (-eyed monster of jealousy)
5. Koishi = Yellow (her clothes?)
6. Chen = Orange (her name)
7. Yuuka = Red (her clothes?).

Normally, I'd guess Kogasa would be because of blue clothing, but her clothes are green here. Still, kind of a clever idea.