HDRP Change color on touch ground on a custom pass shader

Hello, I want to achieve the effect off when a particle system touch the ground, the ground color changes. To make the setup, In the scene, I have a custom pass renderer (injected in before transparent) that works only in the particle system layer and writes the custom depth buffer. Also, I have a custom pass full-screen renderer (injected after post process) that I want to use to compare the depth with the other pass and draw only if it is touching the ground. I started writing a full screen custom shader, but I don't know how to achieve the effect I want, modifying the alpha value of the fragment color.