Reflected sun North tower for sure; looks like reflected sun going onto WTC 6 and the QBNY.com sign from WFC as well. http://farm2.static.flickr.com/1094/126652...f34ec2eb8_o.jpg
Reflected sun if it was on WTC 6 before seems less obvious, but still shows on north tower.
Compare with the Biggart picture below as the South tower is collapsing- looks more scorched than a reflected sun effect as first blush, but IMO could be reflective. No reflected sun on WTC #1's west face:
Similar patterns of light and dark between #1 and #3, but not the same. Second from the top floor appears to have the same windows broken, but the third floor down windows look intact and reflecting in one picture and broken in the other. Hmm. Sun and shadows creating a light/dark contrast on the west face or scorched effect from something else?