All the tanks come in from the factory a light tan in color, and as you know, can eventually reach a chocolate brown depending on how long it sits on the pad in the sun. Any orange tinting should be very subtle. Actually, I think the orange influence is attributed more to what the sunlight is contributing that day whether it be close to sunrise or sunset than anything else.
And it gets worse than that. The super-lightweight ET's, which first flew in 1998, have an even lighter color initially, it seems. See:
http://www-pao.ksc.nasa.gov/kscpao/captions/1998/apr/98pc518.htm
See also:
http://www-pao.ksc.nasa.gov/kscpao/captions/1998/apr/98pc517.htm
http://www-pao.ksc.nasa.gov/kscpao/captions/1998/apr/98pc519.htm
The color of that ET on first arrival was a cream or butter-cream color, with some areas (such as the aft attach struts for the orbiter) appearing white. By the time those tanks are flown, they are darker, but not nearly as dark as the previous tanks were.
Compare the tank color of Columbia being taken to the pad March 23 1998:
http://www-pao.ksc.nasa.gov/kscpao/captions/1998/mar/98pc407.htm
......With the color of the tank when it actually flew on April 17, 1998:
http://www-pao.ksc.nasa.gov/kscpao/captions/1998/apr/98pc498.htm
You can see how much darker it got due to exposure to the sun and elements in about 3.5 weeks. And in some other pics it looks almost like the ET nose got a little darker than the rest of the tank, like the nose got "sunburned".
Bottom line though is that the tanks now are not as dark as they had been. I do not know why. Since they are super-lightweight tanks I could speculate that the spray-on foam itself might be of a different formulation than before, so that it is lighter in mass and happens to be lighter in color initially.
In any case to take note of the graphite ET nose cap and the graphite fairing over the Lox line at the intertank.
For other photos of other missions to check over, try this index:
http://www-pao.ksc.nasa.gov/kscpao/captions/index.htm
- George Gassaway