I think he mostly used color-adjusted GOUT (i.e. the GOUT adjusted to remove its known color bias), but there are other good sources, like the Japanese Special Collection Laserdisc and the -1 preservations, which I think were useful in bringing the warmth back to Cloud City interiors--the GOUT was based on some up-level source that made Cloud City interiors cool bluish in earlier versions.

But the lack of any reference as authoritative as a Technicolor print gave him more free rein to just do "what looked right to him", since there was no evidence stating it didn't look that way. And I agree it looks great.

EDIT: Example of the Cloud City interiors here.