Burgundy to purple?

A friend of mine had lightish-brown hair and came to school the other day with it purple!! I thought she was just doing it for fun, but she said it was supposed to be burgundy. That got me wondering, what could have caused her hair to turn purple instead?

Cheap dye, honestly. You really should do drastic changes with the consultation of a colorist. The reason it turned out purple was the tone of brown under the 'burgundy' shaded the color further and cooled it to a purple. Bleaching first would have gotten the dye to work the way she wanted, but it would also have weakened her hair a lot.