DIAGNOSIS The Low Blue Light Plus or Color Weakness mode is active. Or the picture mode changed.
FIX Press the menu button. Go to Picture. First, check 'Low Blue Light'. Set it to 'Off'. Then, go to 'Color Weakness'. Set it to 'Off'. Finally, go to 'Picture Mode'. Set it to 'Standard' or 'sRGB'.
IF THAT DIDN'T WORK The monitor may have been bumped hard. Go to Picture > Reset Color. This resets all color settings to factory defaults.
HEADS UP Moving the monitor can sometimes trigger the front control button by mistake. Check all your picture settings after any physical move.