Problem is, especially with old movies that have faded--there is no 100% colour reference. Nothing photochemical will last, so there actually is no perfect record. The best single reference, not just available but possible, is a Technicolor print, because it doesn't fade. It's a print, so it has it's own variables, but this is what a restorationist would use. It's what YCM Labs actually did use for 1997 Star Wars restoration colour reference.

The truth is that every colourist eyeballs it to some degree when grading a film because its impossible to get it 100% right. You can start with the Technicolor print as the base, try to collect other film pieces and information, perhaps consult with those involved in the film if you think they are reliable--which, with old movies where you would need them because the prints have faded, they sometimes aren't as reliable as everyone pretends they are--and then use your own judgement to make little tweaks to brightness and contrast. The result is about 95% accurate but if you could get in a time machine and travel back and view the answer print when it was fresh from the lab you would probably find that a scene that you thought had 53 points of yellow actually had 47 points, or something like that. But to your eye it probably "looks" basically accurate.

So, when the restoration of Star Wars happens, they will be looking at the Technicolor print and matching that, perhaps not to 100% faithfullness but it will be the guide to how things should "look".