Can't go wrong with a white shirt and a navy grenadine tie. Since it's summer, you may go with a lighter blue, but either should be fine. You might get accused of dressing a bit boring, but I tend to think that guests at a wedding shouldn't call too much attention to themselves.
I agree with what's being said on here, though, given that its summer, a lilac or pink tie could work great with a light blue shirt. Pastel colors are usually embraced during the summer. As said here, if you want to go with the more conservative option, a navy grenadine tie with a white shirt should work just fine. Please feel free to visit my blog where I recent published an article on dressing for summer weddings, and where you can also find posts related to matching ties with your suits and shirts.