Yup, that's a bad LCD shutter.
The shutters work as a single pixel the size of the entire screen for each of red, green and blue.
Any partial colour defect is not due to the electronics (which only drive the entire single pixel for the respective colour either on or off) rather it is due to a known problem with old shutter panels that fail as yours has.
Often it is a circular blob that will fail, but in any case it is a terminal fault.
Either find another CRT assembly (or cut a good shutter assembly from a bad CRT and replace your shutter) ‐ hardest to source and do.
Or replace the whole CRT assembly and PCB with an LCD - easier but expensive depending on if you DIY or buy a kit.
Or just leave it and live with it - easiest and cheapest.
I keep an eye out for busted up and damaged scopes to strip for spares, _
occasionally you get one with a good CRT assembly, but it depends on how lucky you are.