In the 迪士尼 Franchise, Cinderella's ballgown is blue, but that was not always true (hey that rhymes).
In the 1950 film, when 灰姑娘 got transformed into her gown, it was white. When she looked at her reflection in the fountain, it was blue. When they needed products for this movie, some people thought the white 袍, 礼服 looked too much like a wedding dress, so they watched the whole movie over and over again until they came up with something. They saw Cindy's reflection in the fountain, and thought that was a good color, thus, Cinderella's dress is now considered blue.