 Although the movie is presented primarily in black and white,  particular items are in color and, as such, had to be colored blue  or green on set. According to Robert Rodriguez (I) (qv),  Nick Stahl (qv) (who plays Yellow Bastard) was known on set as  "The Blue Bastard". Yellow Bastard had to be painted blue because  yellow, like green, reacts with the green screen. This causes the  color to spill into the background, making them impossible to  separate.