What you mean is called technical art history. It’s a really interesting topic. But there are lots of questions that even technical examinations cannot answer… in my opinion, in most cases, with a bit experience, it is indeed possible to imagine what the painting looked like when it was new. I mean it’s usually the varnish that gets yellowish, then there is also the krepiertes Blau etc.
In many cases, technical examinations only prove what art historians suspected. And they are so expensive! I would not overestimate the power of technical art history. I mean I am a real fan a technical art history, actually it is maybe even one of my research areas.
We have a lot of experience with real restorations. We know meanwhile quite well how a painting looks if the varnish is removed and/or renewed. So one can draw many conclusions from that.
But in the end, everything is interpretation.