My iMac screen suddenly turn black and white when I'm in fullscreen mode. What could be the cause of this problem?

Hi, once in a while the screen on my 5K imac turns black and white when I'm on youtube watching videos in fullscreen mode. It also exits the fullscreen mode automatically from one second to the next and a warning pops up saying how I can exit the fullscreen mode. Does anyone have an idea why this happens and if this is a software or hardware problem?