She always wears a violet colored vest over her school uniform.
A purple color is a violet color, mainly, for some reason, though I'll let "purple" slide as well.
The reading for 紫 is different from the one you learned with the kanji. To remember this reading, think about how the previous village you went to was all purple. It was the "村先" (むらさき) that you went to. Everything. Absolutely everything was purple.