Because it looked cool. Because black and white with splashes of red were made popular by Sin City. Because it seemed really narrator-y to have black and white and the rose would look all sad and droopy if it wasn't red, so why the hell not, hey look what we can do with
Sam's tie
! Actually that's kind of weird, because that has no red in it normally. Is the rose really blue or black? =O
I figured the monochrome aspect was to contribute to the Rod Serling/Criswell/classic sci-fi/horror narrator aspect of the whole thing, and the red was just to add to the eeriness.
Comments
The film grain of season 3 also helps to that thinking