I have an iMac G3 from the Summer of 2001, and it appears to have an issue with its CRT.
After a few minutes of being on, the entire CRT appears to take a yellowish tint which is extremely visible, especially on blue and white colours.
I decided to go into display settings and colour calibration and turn down each RGB channel individually, and I found turning the blue channel down fully reproduces this yellow tint, and I therefore assume the blue gun is not firing fully or at all in some cases. Is there any way this can be fixed and any reason this may occur? I really do not want to replace this machine as it was a Christmas gift as my family knows I collect these older machines.