Here's a simple scene - box that casts a shadow, one light source, floor, ambient world lighting and everything is in a sphere with principled volume.
I would like to make the selected box appear as if it wasn't interacting with the scene. Ideally just through a material modification - my best attempt in the screenshot does not interact with light, but colors are still affected by the volumetrics.
I have no idea if that's achievable with some smart node setup. If not - any method that doesn't doesn't require two separate expensive renders is good.