Could the discoloration be due to magnetizing so it just needs degaussing?

The ghosting effect looks like the video is going through cheap VGA cable where impedance mismatch creates signal reflections, but this is digital TTL signal in the monitor so it is converted to analog voltage inside the monitor.

Time to open up service manual and start looking for problems. Could be something in the video amplifiers, or power supply in general.

I've had some CRTs have some ghosting effect that are more noticable at certain resolutions and refresh rates. Afaik, the monitor was always like this and I've had it for over 10 years. It was more noticeable against certain backgrounds.

The dark colour in a corner could be a magnet issue like Japael said. If you have the monitor near a large speaker that can do it. I used to have mine near a stereo and the same issue would happen. Try moving the monitor to another area and see if that issue resolves itself. Afaik, most CRTs degauss themselves on power-up so you shouldn't need to do that... but it's possible the degaussing coil failed.