Premature and/or excessive lightening of the paint color, which often occurs on surfaces with sunny southern exposure. Fading/poor color retention can also be a result of chalking of the coating.
Use of an interior grade of paint for an outdoor application.
Use of a lower quality paint, leading to rapid degradation (chalking) of the paint film.
Use of a paint color that is particularly vulnerable to UV radiation (most notably certain bright reds, blues, and yellows).
Tinting a white paint not intended for tinting, or overtinting a light or medium paint base.
When fading/poor color retention is a result of chalking, it is necessary to remove as much of the chalk as possible (see Chalking). In repainting, be sure to use a quality exterior house paint in colors recommended for exterior use.
(image courtesy of The Rohm and Haas Paint Quality Institute)