I'm dwelling over this question myself right now. My SIL is hosting this year. She's uber-stylish and for entertaining she always wears a sharp looking dress. I would prefer to wear jeans, sigh.
I vote to wear something in bright spring colors - "eastery" colors - white is always good. Unfortunately in Boston Easter tends to still be pretty cold, and I'm definetly not ready to bare my legs in a skirt or dress yet. Esp since it sounds like they'll be grilling and that means we may be out on the deck a lot. Hopefully it'll be a warm day.