You do not have to wear beige! The formality of the dress depends on the time of day and type of wedding. You may not have to wear a long dress unless you want to. After you find out what color the MOB is wearing, all other colors are fair game. Check sales, consignment shops, etc. One easy way would be a long skirt and a dressy top. Don't stress over it, everyone will be looking st the bride and groom any way. Enjoy yourself.