I had to look it up in case this was a Berenstein Bears/Shazam bad collective memory: The external tank's orange color is the color of the spray-on foam insula

I had to look it up in case this was a Berenstein Bears/Shazam bad collective memory: The external tank's orange color is the color of the spray-on foam insulation. The first two tanks, used for STS-1 and STS-2, were painted white to protect the tanks from ultraviolet light during the extended time that the shuttle spent on the launch pad prior to launch. Because this did not turn out to be a problem, Martin Marietta (now part of Lockheed Martin) reduced weight by leaving the rust-colored spray-on insulation unpainted beginning with STS-3, saving approximately 272 kg (600 lb). https://en.wikipedia.org/wiki/Space_Shuttle_external_tank https://en.wikipedia.org/wiki/Space_Shuttle_external_tank