The dyes used to make the slides back in the day were unstable. Color is created by the strength of some of the bonds in the chemicals used for the dyes on the film. To make a full color slide, scientists did an amazing feat in making chemicals which were sensitive to different colors of light, then could be transformed during the developing process so that they were not sensitive to light. However, they were not able to make the developed colors completely stable until the mid 90’s. Each color fades at a different rate, with yellow being the worst, red being the best. There were hundreds of different formulations made over the years and each one fades differently. The culprit for all this fade is oxygen and UV light. UV light breaks the color bond, and turns the dye molecule transparent. Oxygen has a similar affect through a different mechanism which goes faster with higher temperature.