A wedding is still a formal event even if the dress code requires casual attire. As a wedding guest you certainly want to look your best but you should keep your "best" respectful to the groom and bride, by dressing appropriately and of course you must avoid anything that outshines the couple.
Here is a list of outfits you NEVER wear to a wedding:
1. A white dress or gown
Unless the dress code requires the guests to wear white you should avoid anything that could remotely be confused with a wedding dress. This includes white dresses both gowns and short ones or dresses in colors similar to white such as ivory, vanilla, off-white, cream, eggshell, champagne-hued shades, golden-hued dresses etc.
No matter how casual the dress code is, wearing jeans to a wedding is absolutely unacceptable.
3. Too sparkly-metallics or anything that outshines the bride
Avoid anything that will take eyes off the bride and put them on you. Therefore, avoid wearing ensembles featuring too much beading or sparkle or anything that is too bright/shiny/metallic. If you want a little sparkle keep it minimal.
4. A too-short or a too sexy-dress
It is definitely acceptable showing a little skin but a wedding is not the place to wear your shortest or lowest cut look or a dress you'd wear to go at a club.
5. The Same Color as the Bridesmaids
This one can be difficult to avoid, but if you know what color the bridesmaids are wearing in advance it would be preferable not to show up in the same color dress. If you know the bride or one of the bridesmaids well, you can ask them, otherwise use the wedding invitation as a clue which is designed to match the colors of the event.
If you attend an Indian or Chinese wedding then you must not wear a red dress since it is the color used by the bride. You don't want to of