DIAGNOSIS The monitor's HDR mode is likely active while you're viewing SDR content. Windows HDR settings can force this. HDR on for desktop use causes washed-out greys.
FIX Turn off HDR in Windows. Press Windows Key + Alt + B, or go to Settings → System → Display → Use HDR → toggle to Off. Your colors will snap back to normal.
IF THAT DIDN'T WORK Check the monitor's OSD. Press the joystick. Go to Picture → Picture Mode. Make sure it is not set to 'Dynamic' or 'HDR'. Set it to 'Custom' or 'sRGB'. Then check Black Equalizer in the Game menu. Set it to 13 (default).
HEADS UP The monitor has an 'Auto HDR' setting in its own menu. Go to System → Auto Source Switch+ → set to Off. This prevents the monitor from forcing HDR on when it shouldn't.