So I did some searching, and tumblr was able to provide me with what I think may be an answer. Cinderella's dress is white, but it would not transfer well from movie to the costume that we see at the parks. Thus, we see her in blue, and the blue was continued on in the form of any other images/merchandise etc that we see today.
According to wikipedia, the dress is shimmering light blue. On the Disney Wiki, it says "In her princess gown, she wore a sparkling white dress with white glass slippers. "
I think the only way we will officially find out is a) if someone contacts Disney and asks or b) we watch when the movie is re released this fall (it's out of the vault as of this October).