What to wear to a Canadian wedding?

I was born in India but raised in Canada, and I've never actually been to a Canadian wedding. We're attending a wedding soon where the bride is Indian but the groom is Canadian, so they are having a double wedding. I know what to wear to the Indian portion of the wedding, but what do I wear to the Canadian part if it's half Canadian and half Indian people? Dresses of course, but what kind? I'm an 18 year old female, so would my prom dress (it's a long fancy midnight-blue dress) be too much for this occasion? Should I go for something shorter? What colours?