Dress Dilemma!

Sorry for the craptacular picture, but I would love some opinions.

I have a summer wedding to attend this week.  The ceremony begins at 7:30 p.m., so it is an evening wedding.  It is located in a metropolitan area, but it's smack dab in the middle of the country (so we aren't talking super-glam NYC or LA or anything).

I know both the bride and the groom.  Joel is the best man.  Their wedding colors are deep purple and rustic copper.

Thoughts?  What should I wear?