This does seem to be a good year for hats, my local debenhams and John Lewises have loads of different styles ands prices. Apricot is dead right, i would like to add, take your own usual summer style and up it one or two dressy notches, look at trousers suits if you are not a skirt person, stick to tailored if you are not a frills and flounces type...then as apricot says, choose the hat that goes. Rather than agonise about colour matching, go for a neutral coloured hat an add a flower, scarf or pin to go with your outfit. For inspiration, it might be worth flicking through one of the myriad bridal magazines that's out there. I bet the bride has a few she could lend you. Last thing is don't worry...you are invited, not your clothes!