Term used to denote that a painted surface has lost its original colour.
There are many possible causes of the discolouration of paint films.
It is often the effect of atmospheric pollutants on ingredients in the paint.  Exclusion from natural daylight may cause yellowing of paints containing drying oils whilst exposure to bright sunlight may result in fading of some pigments.  Some types of moulds or fungi can also cause discolouration of paints.
When discolouration has occurred, there is usually no alternative to repainting but, if a recurrance of the defect is to be avoided, it is necessary to establish its cause and, if possible, to use materials resistant to the condition(s) causing the problem.