my college roomate wasn't greek, but after living with me for 4 years she said all she thought about when planning her wedding was sigma stuff! (I know, she soooo should be a Sigma, but she refuses to alum initate).
Her bf's frat color was dark blue, so her wedding colors were Navy Blue and Plum. (her plum was a deep purple)
She used violets in our bouquets too.
I also had a sister who used purple and violets in her centerpeices. She didn't go over the top Sigma, but she included little things to clue her sisters in to what she was thinking.
I always thought adding something small would be nice, like a purple garter or some other small thing you would know about, but wouldn't make people who wern't Sigma, but involved in the wedding feel left out. I have been to weddings where ther brothers/ sisters all sang or such and it only seemed to make non-greeks feel weird.