"What has come to be viewed as the 'traditional' white wedding dress is of recent invention. Up until Victorian times brides decked themselves out in their best finery; they did not rush out to buy something new for the occasion. The ostentatiously impractical white dress became popular among the Victorian upper classes chiefly for its defects: one had to be well off to afford such an opulent creation, one which would only be worn once."

http://www.snopes.com/weddings/customs/dress.asp

Regardless, what matters is that people still think "impure" if a bride dresses in anything but white. Because of that, where I'm from at least, if a bride is wanting to be known as pure she wears white. Many people who have fornicated, however, wear white just to please tradition, to avoid embarrassment, or just because they prefer it. I have never seen anyone throw a fit because someone wore white who "wasn't pure." This is an observation from a middle class, southern American perspective.