Style inspiration from the Paris ramp
Your PC does not know the monitor is physically switched off, and after a few minutes tries to send the monitor to sleep-mode using the video cable. Physically re-switching it on, and then rolling the mouse to 'wake up' the monitor is quite a rude awakening, hence the garish color tones. Press the menu button on the monitor's casing, and shuffle through the On Screen Display menu until you find the 'degauss' option. Select that, the monitor's display will rigorously shake itself, and its colors will come back to normal.