Apple Multiple Scan 15AV CRT monitor repair

[old_tv_nut]

You're talking about those wrong-colored rings in three corners, right? Ouch!
Are they static or changing?

They may be evidence of a damaged shadow mask. I would suggest getting a manual degaussing coil to see if that will improve it. Such localized problems suggest to me that someone was playing with a strong magnet and bent the mask, or that maybe the monitor was dropped. Degaussing with a strong coil could improve it or prove that it can't be fixed.

Also, look for small magnets (like refrigerator magnets) glued to the bell of the CRT. Some CRTs used them, placed as needed at the factory to fix small regions of impurity. If one falls off, it could create a limited area of impurity.