Why does this appear several times and in different colors? Any tutorial on this subject?
You can adjust the intensity of the bloom by going over its setting in the render properties (camera icon). 
I say around .3 or .4ish
Here's the final render. I added irradiance volume outside the room so that all the objects inside can reflect and bounce lights from three light sources: spotlight, emitted plane (near the walls), and light materials on the cube.
