This is probably caused by an aging CCFL backlight. I don't know the proces for this particular machine, but replacing the CCFL lights or retrofitting the machine with LED back-lighting should address this issue.
It's pretty common for LCD displays using CCFLs to yellow as they age in this fashion, and I have some other mid-2000s systems that have the same thing happening. By the time you notice, it's probably too severe to correct it out using a calibration tool, but I'd be interested in hearing what happens if it's tried.