One of the Color Guns is playing up.
Firstly check the Pins in the VGA cable that comes out of the monitor a connects to the PC if any are broken or bent, then you will just need a technician to replace the cable. Or if everything is okay with the pins, you need to take the Monitor to a Qualified Service Center and ask them to repair it.
Could be a GPU issue but I doubt you're using a Addon card with the CRT. I guess it's most probably onboard so I can't say much about that since they usually just die, no color changing, it happens with older generation addon GPUs when some element on the card die that you might face a color change problem.