(important disclaimer: this work represents me only & not the company I work for.)
I was inspired by an episode of the Powerpuff Girls for this effect. I wanted to create this effect of color spreading in a black and white world, but to do the effect in world space coordinates so that you can see the color crawling over objects instead of flatly spreading across the screen like in the show.
I created this effect in Unity’s new Lightweight Rendering Pipeline, a built-in example of a Scriptable Rendering Pipeline. All of the concepts still apply to other pipelines, but some of the built-in functions or matrices might have different names. I also used the new post-processing stack, but I omitted explaining how the basics of how to set it up, as I think other guides already cover this pretty well, such as this video.