Directors use lighting and color to set the emotional tone of their film, changing these things up during the film to emphasize the way certain scenes should make the audience feel.
Usage of these elements is most effective when they're hardly noticed by the audience at all, so viewers feel the tension, dread or laughter welling up within them without really knowing why.
This episode of 4 Minute Film School by Aputure shows what a difference these elements make to the mood of a film, and how lighting and color are used differently according to genre.
-Via DesignTAXI
Maybe that's why actors were invented.