Believe or not, it took zero effort to make the background of that shot of my purple rim green Monti cap. look like that. That coral was sitting right below my 400w MH. With that much light bouncing off the subject, the exposure alone resulted in the background being dark. No changes to levels or curves. No layer masks. Nada.
That there was nothing immediately behind it also helped. My overflow was probably 4" behind it but not in the footprint of the MH light, so it's delightfully obscure.
Ken