IF.....

I didn't put those white flags on the Brooklyn Bridge. But if I had, this is how I would've done it.  Buy two 11'X 20' American flags. Place flags in two large plastic garbage cans. Add  5 gals of bleach to each can. After one hour remove from can and hang dry. If fabric has not completely turned white, repeat process. With another person, fold flag properly into tight triangle and place in backpack. Buy 6 large disposable food baking pans, placing these also in a large backpack . Get ahold of a couple of trusted friends. Drive to Manhattan at 1:00 am. Park on Manhattan side of bridge.
   Wearing camo clothing and facemasks, climb bridge cat walks leading to flags. Wearing safety harness, clip to cable and scale locked gate. Once lights are accessed, cover with pans and lower bridge flags. Fold bridge flags and put in backpacks, replacing with bleached flags, raising them in place. Scatter empty bleach bottles and get the hell out of there.

PS
The Trading Post has those big pans.