Weathering White Paint

Depends on what color it's fading into...you could add a little grey, or even tan.  I find dry brushing with a light grey or tan to be most effective.

White is a very hard color to get just right. Work slowly, and build in progressively darker layers. 

I also find weathering powders are pretty effective on white.

White weathers to dull white, because titanium dioxide (the pigment used) is one of the most stable compounds on the surface of the planet. Whites tend to weather away, revealing the colors underneath. A light treatment (dry brush or pastels) with a light gray would be appropriate, unless there is a definite color under the white, in which case, use that color.