Attached to this reply is an analysis using a better lighting calibrator than the police car, a sunlit and self-shaded piece of Flight 77. It provides a clear example of the relatively bright tone of shaded Flight 77 skin, especially relative to the dark trees. It still looks like a white (or maybe light blue or gray) object. Placing a reduced piece of the piece of Flight 77 up against the trees shows that it is far more like what the ASCE report (see page 13) identified as Flight 77 than the pointed-to dark tree-line protrusion, which if it were Flight 77 we should expect to look as bright as the self-shaded debris sample.
Ian, although I'm no photo-analysis expert, I know you can't directly compare the apparent brightness of the debris piece with that of the objects in the security video. The security camera is primarily focused on an area of directly sunlit concrete, which will cause the camera's aperture to be smaller, and thus shadowed objects in the video to appear darker. Conversely, the photographer of the debris piece undoubtedly set the camera's aperture and shutter speed to correctly expose the piece's shadowed side. Therefore, the photographed piece is likely to appear much brighter than the aircraft's fuselage and vertical stabilizer appear in the video.
Also, you had earlier suggested that the object we take to be the tail fin might be a light pole that is in the process of being knocked down.
Here is a measurement I made using Mike Wilson's SolidWorks model, and
here is the view of the model from the security camera's point of view, along with the video frame for comparison. I measured from the tip of the vertical stabilizer to the last light pole that was knocked down. Note that the z-axis represents the aircraft's approximate ground track. The light pole is over 100 feet farther back along the ground track than the object in the video frame.