Degaussing operation after power on (that thunk sound) should fix such things. This is caused by magnetization of the mask in the screen or nearby magnetic field (i.e from non shielded speakers). Sometimes you need to power on the monitor several times (with few minute breaks) to fix that. Also changing positioning of the monitor while it is running can cause this, in which case another degauss operation is likely needed though effects of that should be far more subtle.

You could also take a risk and use a strong magnet on the case and swipe back from the front side of the case (where the discoloration appears) to the rear where the tube ends. I have fixed a couple of TV's with this method, but it might require a few tries and you might experience stronger discoloring at first. Also, I don't take any responsibility if you make it worse, but if it's already ruined then what's the harm?