Add a post process volume to your scene if you scene does not include one already. A post process setting is like an instagram filer for your game.
Make sure the post process volume has the “Unbound” setting checked, basicly this will tell the filter to apply to the entire scene, not just to the items inside of the volume
Create a material (your filter), and change the material settings as follows:
Then in the material take the SceneTexture PostProcessInput0 node, connect it to a 1-x node and then connect that to your emissive input.
Apply this material to your post process volume
You should check the article I posted in the previous answer for more info