When paint starts to prematurely lighten. This can result from paint chalking or from being exposed to intense light for moderate to high portions of the day, such as southern exposures.
A few causes of this issue are using low quality paints, using wrong interior and exterior grades of paint, using paints that are prone to ultraviolet deterioration, and painting before surfaces are properly cured. A couple other causes are tinting paints that do not need it or modifying paints to lighter or medium bases.
If chalking causes fading go through the chalking solutions above. If the fading is caused by an alkali “burn,” be sure to use a primer with alkali-resistant properties.