The last flight-qualified space shuttle external tank in existence is now in Los Angeles and moved to its new home over the weekend. I noticed that what appears to be the metal skin under the insulation is also dark orange/rust colored, very similar to the insulation. I'm guessing it's paint on the metal.
As explained in this answer the dark orange/rust color of the foam insulation is it's natural color, and it darkens after exposure to the sun.
In the first photo (found in Twitter) you can see the metal tank exposed through the funny-shaped hole in the insulation. Not identical colors, but very similar. There's a good video in the article also.
I'm wondering if the metal skin was painted a similar color for aesthetic reasons, or for another reason. If it were white underneath, then "missing foam" would be much more apparent. That could be considered a good thing or a bad thing.
Photos are circa Saturday, May 21, 2016 in Los Angeles, California. First two from here.
This one is from the NPR story