I was told many years ago that there are three colours one doesn't wear to a wedding: white, so as not to upstage the bride; black, because you're not attending a funeral; and green, because that is the colour of envy.
However, this seems a little old-fashioned and very few people on this thread (or indeed, IRL) seem to be worried about it!
I think the dress looks very nice and the shape is certainly appropriate for a wedding guest.