AFAIK the only "rules"/customs are the bride is the only one that gets to wear white. Other people should avoid the bridal colour.
Last wedding I went to was some time back now - so I wore a "best" dress I had anyway (ie long length/long sleeves/high-ish neckline/Laura Ashley). It was multicoloured. I figured it covered all bases - not white/not sexy (and I was prone to wearing low necklines - but that dress was a bit more cover-up than that).
I think white or "sexy" are the only faux pas these days for female guests at weddings.
I went to a wedding at Easter and I really panicked about my outfit . It was absolutely ridiculous. I ended up buying a new dress ,shapewear and new shoes . Now I think that the dress was a waste of money and I'm not sure that I'll wear it again . Sorting out my wardrobes recently I found several dresses and shoes which would have been perfectly acceptable . At the wedding- Very few women were that dressed up . Several looked as though they were having a casual day shopping ! If you like your dress wear it and stop worrying-just enjoy the wedding .
I'm attending a friend's son's wedding next July and want to wear a red dress I have worn to several occasions, including my niece-in-law's (is that the right term?) wedding a few years back. Looking again, everyone else seemed to have got the memo and is wearing pretty muted colours and mine kind of stands out.
Should I stick with my red dress anyway, or should I really be toning it down a bit?