Cutoff vs emissions
 

I have a color tube on the tester that is showing extremely weak cutoff on all three guns, but good emissions on all three.

Is the cutoff the “first thing to go”? How is it possible to have almost non existent cutoff but good emissions?

The data is certainly backed up by image quality on screen which has lackluster contrast. Incidentally, the bias switch (on in this case a late 60s Sears Portable) has to be set at max for a decently viewable picture.

CRT testers operate the tube like a triode. The emission reading comes from the current between the cathode and the first anode. The cutoff is the grid bias needed to cut off the triode.
If there is gas in the CRT, then the current is partially due to ionization of the gas and that is not controllable by the grid. So this may be what is happening.

Can you see the getter flash?
If it is turning whitish then that would confirm this.

That's a good question. No getter is visible, I think without removing the chassis partially. Will report my findings later, but good explanation, nonetheless. :yes: