Appropriate Attire for a Wedding

What is the consensus on appropriate attire, generally, for a guest at a wedding? I have two coming up in several weeks. One will be less formal than the other. The groom in each case will be wearing a suit--not really formal dress. I'm thinking a suit is probably right for me, but I'd like to wear a less formal gray glen check suit rather than a navy suit (I have only the two). Unfortunately, the gray suit is a somewhat heavier wool than the navy one, and may look wrong for a summer wedding. Is weight of wool more important than color (lighter vs. darker) in this case? Also, what about shirts? Can I go to colored shirts, or is a white shirt the norm? Finally, shoes--can brown lace-ups work, or is black preferred? I'd appreciate any help you guys (and ladies) can provide.

Vancouver