Either colour would be completely fine and long as you don't over do it with matching jewellery or its going to look a little too much.

If you wear flats and are uncomfortable in heels then wear flats...Other wise you will spend the night thinking "my feet hurt" and its easily seen on your face. I highly doubt anyone will be looking at your feet thinking "omg, she wore flats!?!".

I wear flats a lot since breaking my foot a few years ago and I've never ever had a problem and having been to a few weddings wearing flats I'm usually the only one with my shoes still on at the end of the night.