This provides some explanation about the flags:
Cops collect DNA in Brooklyn Bridge flag caper - CNN.com
"New York police have collected DNA evidence in the case of nighttime acrobats who
replaced two American flags atop of the Brooklyn Bridge with two white flags, according to a law enforcement official with knowledge of the investigation Friday.
The
swapped-out bleached flags were spotted by construction workers at 5:30 a.m. Tuesday."
"The
oversized white banners -- American flags that were pre-bleached -- have prompted investigators to
talk with flag manufacturers and check websites for anyone who may have recently purchased large Stars and Stripes, according to the official."
From those descriptions, it appears that the vandals brought large pre-bleached white flags with them to the bridge. Then, they swapped out their flags for the regular flags that were mounted on the bridge.
In that case, the vandals could have bought cotton flags, bleached them, then took them to the bridge, ready to swap for the nylon flags. They didn't do any bleaching at the bridge itself.