No, it’s definitely not a degauss issue. I thought that at the time and have tried a degaussing wand several times but it makes absolutely no difference.
If you tilt the monitor whilst hooked up to a test colour pattern, you can see the colours change.
I’ve had a similar issue before with a colour vector monitor. The monitor was factory set to horizontal, but rotating it vertically gave me colour purity issues. In this instance moving the machine to a different location pretty much solved the colour issue.
In the case with my 17” LG/Philips tube, no amount of moving drastically improves the picture (apart from standing the tube vertically upright in a horizontal position - then the colours are perfect). Great for a candy cabinet, but no real use for a lot of cabarets which tend to have the monitors mounted at 45°