Your slides and photos will fade for a variety of reasons. Dyes will fade because they are in the dark ( termed "Dark Fading") and dyes will fade because they are exposed to light (termed "Light Fading), meaning light being projected through the slide. Dark Fading occurs when your slides are not exposed to light. It is caused by a temperature and relative humidity reaction. Dark Fading will occur even if your slides are stored in the light. All dyes have a limited lifetime because they break down because of temperature, light and chemical reactions to materials within the dyes themselves. Prior to the mid-1980s, the Cyan dye was particularly unstable. Reacting to this, the film companies improved the rate of cyan fade. Light fading is caused by exposure to high intensity light such as when a slide is shown in a slide projector.

While some brands of slide films, such as Kodachrome, have good dark fading stability, projection time should be kept to a minimum. Accumulated projection time and not the length of a particular showing of a slide, is what is important. If a slide is projected many times, this is particularly important. Glass mounts may actually increase the rate of fading rather than protect the slide.

Article Credit

BLOG - Why do slides turn Red or Blue?