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In the TV series Supernatural, almost all episodes have a darkish colour scheme. The only exception is for episodes partly set in another universe, e.g. S05E08 "Changing Channels" where they brothers were put through a number of TV shows, or S06E15 "The French Mistake" where they are thrown into this world. The effect on the viewer is that the Supernatural universe is darker and "grittier" than our own (and other series).
Is there a technical or colloquial term for this effect?