I think lightsaber crystals should optically degrade over time, with the crystal darkening until it eventually became red.
The Jedi, being largely pacifist, would not use their lightsabers often. This would preserve the colors of the crystals. Some yellowing would still occur of course, requiring new Jedi to build their own sabers instead of using old ones passed down from generation to generation.
For the Original trilogy, this would mean that Obi-wan’s saber would be yellow from years of use. Anakin’s would be in better shape but still wouldn’t be bright blue. Vader’s would of course remain red.