Masking Decals in Post Process

Is there any way to create a mask for decals only in a post process material?

I am trying to create a basic “make the whole world black and white, except for ____” material where decals are the only thing rendered in color. Is this possible? I know I can’t use custom depth since they aren’t meshed but figured there must be some kind of SceneTexture thing I can grab or use to create a simple black and white mask.