I'm new here so this has probably been gone over like 10k times but I searched and didn't find any so here it goes.
Ever since nasa decided to not paint the external fuel tank to save 300 lbs. weight the foam used has weathered from UV rays from sun lite before a launch. The new fuel tanks are white when the foam is new and gradually turn orange, the color you see them at launch. This orange color represents the external layer of foam reacting with UV rays causing it to decompose. You would think nasa would store these tanks covered and in the dark but not so. In the past nasa has stored these tanks outside in the Florida sun, some even had bird nest in the foam and had to be repaired, I don't know what they do with them currently but on the next launch,, see if it's orange. The tank on the last launch was made in 2002