Color fading is caused by spontaneous chemical changes in the image dyes of color films.
Many older films have taken on a distinct purplish cast, caused by the rapid fading of the cyan and yellow image dyes. Negatives, interpositives and prints are all affected in the same way. This was once a full color image before the dyes faded. Cool and moderately dry storage is required to ensure a long life for color film. Manufacturers are working hard to improve this aspect of their products, but, until they do, the only way to guarantee centuries of useful life for color films is to keep them in cold storage.