That is by most chance your monitor. Is it old? My brothers monitor turn purple or yellow sometimes, this is due connection problems with the cable from your monitor to your video card. In his case, he put the monitor to close to the wall, so the cord from the monitor bent downwards and put pressure to the soldering inside the monitor. After a while, the connection got loose.
I would suggest you to take your monitor to a technician, it's pretty easy to fix, if you figure out HOW to open to monitor (that's the tricky part)