I have worked on many TVs... I would say hundreds over the last 7 years.
Hi,
Sorry for your sets color problems.
Durring the time that I have worked on TVs, I have only seen THREE bad CRTs that had not been damaged... Shot in a rage, dropped from a table...etc.
I would still advise Checking into your sets Video Processor IC. If you can look up it's specs (There are several on-line sites to do this. I could send you a link), find it's R,G,B inputs and test the capacitors in their circuitry, either by substitution or with an ESR meter. This would probably see your set recover.
I say probably because I have seen open resistors, and shorted transistor color amps on the CRT board (actually on the neck of your CRT) cause this as well. Also it possible that the actual Video Processor IC has gone bad, although I doubt this as it took a while for your picture to deteriorate.
Good Luck.