I need your help!

Whoever told tu brown hair doesn't dye well purple, they weren't saying truth. my friend naturally has mid-brown hair and she dyed it a subtle purple color, and it looks ah-mazing. That kind of thing you're talking about would probably apply to people with say black hair trying to dye it light blonde.
on the side of the dye box it should mostrar a color spectrum of how it's expected to look.
If tu want purple hair, go for it. go for what ever color tu feel like, then tu will be less likely to be disappointed. don't worry about what anyone else thinks, cos I've seen it on brunette people and it looks fine :)
It would help if when tu wash your hair the día before tu dye it, that tu use a deep conditioner to make it in good condition, then the result will look better.