Colour print fading.
It is worth noting that since the 1980’s technology for colour print stability has greatly improved.
With the advert of Digital photography and inkjet prints, it has reached a stage that colour prints should last a lifetime and longer if stored in cool, dry conditions.
Prior to this the chemical reactions in colour photographs cause fading within a few decades. A common effect is for photos to get a strong red cast. Often this is not to difficult to reproduce and fix.
The 2 main types of colour prints are;
Chromogenic (or traditional photo lab prints), and Inkjet prints.
Ink jet prints are considered stable for archival quality (up to 200 years), however colour prints made before 1985 have usually faded to some degree.
Modern chromogenic prints are better but still not as good as inkjet.
This before and after example of a pre 1985 print, shows how even a badly faded colour photograph can be resurrected with restoration
Another reason for fading is incorrect processing at the time. Sometimes portraits made for instant portrait shopping centre or restaurants, were processed for speed rather than longevity and had less washing (removal of residual chemicals) than was required. Sometimes these images fade unevenly making restoration difficult.
The only way to guarantee non fading of traditional colour photographs is to keep them in cold storage. This is because heat is one of the reasons for fading. It is interesting to note that a photo will continue to fade even in total darkness, if exposed to heat.
James M. Reilly- Storage Guide for colour photographic materials.