Historically, CRT monitors used to have separate red, green and blue
intensity adjustments to cope with the fact that the separate guns age
at different rates. The potentiometers for making these adjustments
were sometimes made accessible without removing the case, sometimes
not. In any case a large busy IT department might well decide that a
monitor with degraded color balance was due for replacement, as it was
not considered cost-effective to do the adjustments manually.

More recent models had a microprocessor to handle control and setup,
and some Sony Trinitron monitors, including Dell badged ones, featured
"digital color return technology" which was intended to enable a return
to original factory-shipped color temperature and luminance if desired
at any point over the lifetime of the monitor.

The operation needs to be performed with the guns warmed up, and it is
locked out for about 10 to 20 minutes after switch on. During this time
an "Available After Warm up" message is displayed if Color Return is
selected using the OSD menu. After this time use of Color Return is
possible.

One way to tell if a Trinitron monitor had microprocessor control was
to see if Color Return is a menu option.

Obviously, if the tube is too far gone, perfect balance and luminance
will be impossible to restore, but even with quite old kit, 7 to 10
years, people report acceptable results after using this feature.

I have heard stories of people who know about this retrieving Trinitron
monitors from dumpsters because university or business IT departments
have replaced them for being "too red" (or blue or green) because they
were not aware of the Color return feature.