There is no law or rule stating that wedding gowns must be white.
White has nothing to do with virginity or purity (traditionally speaking, that color is actually blue). It's merely a fashion trend that caught on. A trend that started as a showcase of one's wealth (because an elaborate white gown could not be re-worn to other functions, and at the time the trend started they were made of hard-to-get materials).
Plenty of high-end and well-known bridal designers offer gowns in shades other than white or ivory. You could also look in an upscale department store or clothing boutique for an evening gown in your desired color. A "wedding dress" is simply a dress worn to your wedding, no matter where you buy it or what it's labeled as.
If your family isn't paying for the bridal gown then they don't get a say in what it looks like. If it'll cause tension with them pre-wedding, just shop for it without them and tell them you want it to be a surprise. Or learn how to shut them down: "Well, [shrug], sorry you feel that way but I really like this gown and I'm buying it. If you're so angry about it that you'd rather not come to the wedding, I'll understand." End of discussion.
If they're not paying for it then you don't have to convince them that you're right. Because (a) you don't have to, and (b) it won't work anyway. They can complain all they want, but nothing is forcing you to listen or respond. Just ignore them.
Plenty of celebrity brides (and "regular" brides, like both of my grandmothers who wore non-white day suits they already owned, because it was the 40s and they couldn't afford otherwise) have worn non-white gowns ... Sarah Jessica Parker, Jessica Biel, Anne Hathaway, Princess Alice Duchess of Gloucester, etc. Use Google to see for yourself.
http://ultimatehistoryproject.com/before-the-whiteout-wedding-dresses-and-grooms-outfits.html