How to add an area that turns a sprite gray

Unity Engine
1

I’m using unity2d and i want to create a area that turns a sprite gray but only the part that is in the area like this image

6122678--667247--Example.png
6122678--667247--Example.png800×600 2.84 KB

2

I think you should use 2 cameras, and only show camera 1 in the area with a saturation filter set to zero, and set the second camera to display only the background without the filter and so, in color