Orange and purple is a fantastic combination! If you want a tiny burst of color added to that scheme just to create some edge, then a lime green would definitely be a suitable option. Example - orange roses and dark purple hydrangea would look amazing with green button mums thrown in.
I agree with the other posters - with such a striking color scheme, I would use a neutral to ground it. Black is fantastic. It can be worn any time of day, dressed up or down and everyone can use something basic black. Put your BMDs in black A-line, knee-length dresses (Ariadress.com is a great vendor) and let them choose their own necklines so they'll be comfortable. Then, select a sash that is either orange or purple. Or put everyone in one color and your Maid of Honor in the other color.
So many choices!
If you don't like black dresses, a celery green dress would be darling and then have orange & purple bouquets with slight touches of green baby hydrangea.
Have fun!
