Just a crazy idea to make the most of what is available to us.
Supposing you take some of these faded prints that are held privately. Scan them in. Convert to black and white - use that as luminance data.
Now take a LD scan, and use it’s colour data - combine it with the luminance from the print scans.
Since colour resolution doesn’t need to be as high as luminance, maybe you get a decent result.

Then maybe the fact that the prints have faded or gone green or whatever, doesn’t matter.