DYE FADING.
All dyes used in photography fade with time. Many of the dyes selected for early additive filters are reasonably stable but those dyes developed in some way during processing fade to considerable extents. The dyes used for toning, Prussian blue, uranium ferrocyanide and the basic dyes used for the two colour processes like Cinecolor and the numerous duplitized print processes, all fade, although Prussian blue and perhaps uranium ferrocyanide tend to darken and desaturate until almost no colour remains but the image is darker than the original.