How to highlight area that is NOT in the field of view rage of a GUN/CCTV Camera

Hi , I want to highlight an area (in red color) that is NOT in the range of field of view due to some obstacle. I found tutorials that help to highlight the area inside the field of view but I need the inverse of that i.e Area that is not in the field of view should get highlighted.

A small image is attached. In the provided image I need to highlight Red Area. Square is my obstacle, blue circle is my player and Yellow circle is the weapon range.