Trying to highlight part of an image with a masked plane

THX1139 Posts: 545 Just Starting Out

In the past, I somehow used a plane with a mask to highlight part of an image to make it look like it's flashing. I can't understand how I did it or reproduce it and I can't find the information by searching my previous posts. In the below screenshot, the bright green area is somehow being generated from a masked grey plane?!