Post-processing overlay effects

Is there a way to make image overlay effects similar to what Photoshop uses? I want to be able to duplicate the current camera view and then use a screen overlay effect. I found this tutorial and got it working but am unsure how to alter the shader to process this kind of image effect that I'm trying to create.

https://docs.unity3d.com/Packages/com.unity.postprocessing@2.1/manual/Writing-Custom-Effects.html

Here's some code I found for possibly creating the screen effect, not sure how to use this to calculate though

 fixed4 Screen (fixed4 a, fixed4 b)
 {
     fixed4 r = 1.0 - (1.0 - a) * (1.0 - b);
     r.a = b.a;
     return r;
 }