The above recommendations are good starting points, but keep in mind the color of the ET is kind of a moving target, so to speak. Aside from the fact the basic color has varied a bit over the years, it can also vary on each individual ET depending on whether the foam was applied recently or some time ago. The color changes with the age and the elements.
The last time I painted an ET, I mixed Testor's "leather and "radome tan" to get a color I thought was close. I can't recall the ratios I used, though. I do recall that for the ET's lighter-colored ablative coating used on the ET, I used the same paints, but mixed in the opposite ratio. (I just can't remember the ratio, though.)
Once you get it within a certain orangy-tannish-rust color range, you should be ok. Reference photos can be helpful, but the color in them can also be off a bit; plenty of times I've seen the same photo reproduced in different formats and they'll show the ET color differently. That means you have to keep in mind the limitations of the reproduction quality of the reference photos you're using.
Then there's the whole issue of what lighting the photo was taken in....
I've seen various "official" sources describe the ET color as everything from "rust" to "apricot." For what it's worth, a publication put out by Rockwell International in 1983 and titled "Shuttle Model Information" (with artwork by Stan Jones) describes the ET color as "nongloss light sienna-ochre (autumn gold)."
Good luck.
David Hanners
Minneapolis, MN