Is there a custom light/fog for cancelling a filter?

I am working on a project where the background scene on a 2D game is colored, but we are applying a grayscale filter over it.

There is an attack the player can perform that is a colored beam. I want that beam to emit a light-like effect (strong up close and fades out) but where it overrides the grayscale filter to show the colored scene underneath.

This effect is temporary and attached to the attack beam game object. It should fade out from the true scene color to the grayscale filter.

If it doesn't exist, then I am likely going to need to code it into a script to attach to it myself. If anyone has any suggestions for this Plan B, that would be helpful.

My current plan, if I have to make it, is to take the equations and methods for how light works, but alter how RGB values are calculated to instead cancel grayscale.