IIRC it was a mix of a color-balanced GOUT, Puggo Strikes Back (16mm print), and Harmy’s eyeballs. I believe there was an incomplete 35mm source from Poita toward the end of this that may have also helped.
For example, while I think color-balanced GOUT was the major reference, there were a few instances where the GOUT diverged widely from PSB and other theatrical prints (the warm orange glow sometimes seen in the Cloud City halls was a cool blue in the GOUT). In those cases, PSB/prints were the reference. I believe the prints were similarly used to “settle” the issue of bluish ice on Hoth, not that people don’t still complain about it.