There are several possibilities here including the Picture Tube screen voltage, bias settings, high voltage, and the picture tube itself.
Consistent with the age of the set and if the voltages on the tube are normal, then the picture tube is the fault. The picture tube has three color guns that gradually reduce in emission over time. Anyone of the three primary colors of red, green, or blue can start to spread out from the whites and it is sometimes called flagging.
The only solution other than replacing the CRT is to run the user controls at a lower setting. It’s been to long and I don’t remember if there are adjustment knobs on the inside.