Regarding color correction, if you’re interested in the topic, you should check DrDre’s threads and color correction theory and methods very carefully. I was skeptical at first but have been convinced. Color fade can be modeled and reversed mathematically, and can be done for the most part without using any judgment calls, as long as you can find good candidate frames in the shot. Because of this, when anyone uses DrDre’s tools to color-correct faded images, they get very close to what can be called the original colors with a good deal of certainty. So when DrDre does his correction, it’s 95% math, 5% finesse on top of that. How do we know those are the theatrical colors? Because that’s what the math says they are – if they’re not, the only explanations are that either color theory is wrong, DrDre’s code fails to correctly model color theory, or you’re using the tools wrong – because seriously, it’s just math. While white balancing against the soundtrack is a clever idea, it doesn’t account for fade with nearly the same level of accuracy as DrDre’s tools (in short, because color correction is more than just white balance), and doesn’t deliver good results.
When you look at DrDre’s color corrections for Star Wars, Empire, and Jedi, it’s very easy to see that Despecialized is currently a closer match than 4Kxx for all films. I say “currently” because, as people have said, 4Kxx is always getting new color corrections (and DrDre himself is working on some of them), and will certainly one day surpass Despecialized in terms of theatrically-accurate color.
So with one caveat, we can, in fact, know what the theatrical colors were, with a good deal of certainty. And in fact we already do know, for the most part. The caveat is that different filmstocks may have different color characteristics – for example, the Technicolor prints have a yellowish tone not present in other prints, etc. But these were slight color differences compared with the huge variance we currently see among fan preservations. Because the colors of some of the fan preservations are, to be blunt, way outside the range of what could ever have possibly been correct (specifically, they have some of the same overly-magenta cast that has plagued Star Wars home video releases since the THX releases, which is probably because the color correctors were trying to make it look like their old VHS tapes, and succeeded).
For example, want to see what the theatrically accurate colors for Jedi were? Easy. You’d be correct if you noticed it doesn’t look anything like any existing 4K83 release. Empire? Here. Star Wars started off as matching someone’s manual regrade, but I think has matured into using the color correction model more exclusively, which is why I’m linking to latter posts in that thread.