I forgot that the OP was in Australia.
Honestly, it's really, really common here for bridesmaids to be very matchy. Matching hairstyles are extremely common. Having your hair done all together is extremely common. It's an unusual wedding that doesn't have a matching bridal party. Truly, I would have been to 15 weddings in the last 8 or so years, and one had bridesmaids with different hairstyles
OP - I'd be really annoyed about you changing your hair so dramatically just before my wedding. It really would ruin the pictures, because as Toots has said, the purple hair will stand out. And if you hate it, and never have it done again, it means that for two months, right at the time of my wedding, when I'd like a matching bridal party, as has traditionally been the case here, you've gone out and got a really different look for only that time period. It's just not cool.