Question How do I go about creating a shader that turns a pixel the oposite color of the pixel behind it?

I am working on a crosshair system and I want the crosshair to always be visible
I thought a good way to do that would be to have the pixels of the crosshair always be the exact opposte color of what is behind the crosshair

Using URP and not very used to shaders or HLSL code

 

https://forum.unity.com/threads/invert-colors-shader.205244/

 

Thank you