Fair enough op I didn't want to assume you were British and that therefore were already aware of the cultures and customs here.
Even in other English speaking countries wedding customs can vary greatly.
Navy - I would check with the bride.
Generally an inoffensive colour I wear a lot myself (I'm a redhead so blue suits me and a lot of colours don't) but as it matches part of bridal party and is a dark colour close to black not everyone likes it for a wedding.
If the dress is not completely navy especially if mixed with white/cream it will likely be ok or you could use lighter coloured accessorising.
Depends how dark a navy too.
All that said I wore a plain navy dress to then boyfriends sisters wedding, I was young and didn't know the rules so well then, but also I did wear with cream accessories - cream and navy shoes (fake Chanel style pumps, cream body navy "toes"), cream fascinator and bag, fake pearl jewellery, even cream tights! I know! It was the 80's and I THOUGHT I looked so grown up!
I saw those wedding pics many years later and I looked ridiculously old for my age not helped by the tight perm!
I was 17 and looked like I was TRYING to look 40! 
Ah well we live and learn