It means that your monitor is not reproducing the blue component of the video signal.
Your computer sends images to your monitor as a combination of red, green and blue colours (RGB). All RGB colours in a balanced ratio produce white or gray colour. If Green is missing or weak, the picture is purple; if Blue is missing, it's yellow; if Red is missing, the picture is greenish-blue.
Since the fault on your monitor shows up some time after powering on, and since you've checked all external connections, it must be a heat-related loose contact somewhere inside it, probably a dry solder joint in the blue signal line.
It shouldn't be too difficult for a good technician to isolate it - an electronics technician, not a computer hardware tech. Chances are it's on the CRT base board, i.e., the small PCB (circuit board) fitted with a socket to the rear end of the picture tube.
Theoretically, it could also be on your graphics card, but this is much less likely.