|The judges from Project Runway will not be taking notes on anyone's outfit
Wedding night attire
Ladies who want to avoid being mistaken for a bridesmaid should stay away from royal or electric blue. However, this is more for your sake than ours; if it looks good, wear it, regardless of color.
As far as how formal/informal to dress, it is (after all) a wedding; vintage attire is encouraged for those so inclined, and cocktail/semi-formal for those who aren't. Of course, this is Seattle, so if guests manage a level of formality exceeding jeans and T-shirts we'll be pleasantly surprised.
Just one last reminder for the Southerners: nighttime here is going to feel cold. Whenever Nick complains about the weather, his dad uses the same Mark Twain quote about how "The coldest winter I ever spent was a summer in San Francisco", even though Mark Twain never said that. But it's true—it gets cold at night year-round! The joke here is that summer starts on July 5th. September lows are usually in the low 50s or possibly the mid-40s, plus there will be a little bit of wind. Even during the day, the usual high is 70 degrees and on a cloudy day it might be 60 degrees.