I was just in Ann Taylor the other day and there were some nice light/dark blue/navy combo dresses. They could be appropriate for summer (light blue) or fall (dark blue and navy) based on color and also they were a decent material that isn't gonna be really hot or wrinkle that easily. I don't know how that would look with your date's suit, but I think it may go good.
I guess I feel like if you have to go with a solid color, I'd go with a lighter one. The wedding is in the morning and it's still summer. Now if it were August, I may go for a darker color, but most people who have that type wedding (summer morning) generally would be going more for a lighter color feel. Not that I would know since I'm gonna be single forever
jk But that's my $0.02.
Tara
I have a Phi Mu castle...