White objects are not perfectly reflective and whiteness is not so much a function of reflecting them well, but reflecting them relatively uniformly in the visual range. Not every white object reflects UV.
Black plastic lasts longer because 1) the UV is absorbed by the pigment, not the polymer and 2) the pigment limits UV penetration to the surface.
Black plastic lasts longer because 1) the UV is absorbed by the pigment, not the polymer and 2) the pigment limits UV penetration to the surface.
Here is a good doc on UV weathering of plastics and materials (such as carbon black) to absorb UV. http://www.cabot-corp.com/wcm/download/en-us/sb/UV%20WEATHER...