Alfonso Cuarón always uses green (he even auditioned green eyed girls for A Little Princess (1995)) as the main color of his movies. He uses red and orange (complimentary and opposite colors) to make some remarks.
Ethan Hawke later dissed the film, calling it "a lousy experience". He added that he felt he had been talked into making the movie, something he realized about a month into shooting which was then too late for him to pull out.
The trivia item below may give away important plot points.
The green color appears in any unexpected forms, ways and objects along the entire movie. In latin traditions, the green color means hope, and the spanish title asigned for this production is 'Grandes Esperanzas'. 'Esperanza' is the spanish translation for 'Hope'.