What jewelry would look best with this dress for my Bridesmaids?

If I were you, I would skip the necklace idea entirely. Instead, opt for blingy drop earrings that dangle. They will elongate your bridesmaids' necks and won't compete with the neckline whatsoever. Additionally, you could do cuff bracelets or a single chunky, sparkly bangle. I also saw on Pinterest initial rings for your bridesmaids that you could give as bridesmaid gifts - rings that they can put on on their right hands as a reminder that they have been and always will be your "right hand girls."

I personally love gold and think it's making a real comeback. It especially looks nice with red and other warm colors - great for fall! As for gemstones, garnets, rubies, and other red stones would look amazing. Don't break the bank looking for jewelry though. Try Etsy, small local boutiques, Charming Charlie, Forever 21, etc.