Women Painters.com: Paint Issues and Fixes:

FADING / POOR COLOR RETENTION

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.

An example of fading or poor color retention

image courtesy of
The Rohm and Haas Paint Quality Institute

Possible Causes:

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.

Solution

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.