Director 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.
In addition to co-writing, performing, and producing the song "Siren" especially for this film, Tori Amos provided the vocalizations (whistling, breaths, et cetera) that can be heard in the film's score.
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 assigned for this production was "Grandes Esperanzas". "Esperanza" is the Spanish translation for "hope".