Ironically, the whole idea of Bridesmaids came about to fool evil spirits or anyone who might try to steal the bride away, so Bridesmaids were dressed to look as much like the bride as possible!
And the concept of wearing white at your wedding became fashionable after Queen Victoria wore it, its not traditional its a trend that never went away! Before Victoria a woman of low to modest means wore her best dress to get married in, the upper classes would have a gown specifically for their wedding but they were the outliers. Clothes were very very expensive so if you could afford a gown that was only worn once it was more about parading your wealth than anything else.
Its just got bloody daft now.