Source : WordNet®

green card
     n : a card that identifies the bearer as an alien with permanent
         resident status in the United States; "he was surprised
         to discover that green cards are no longer green"

Source : Free On-Line Dictionary of Computing

green card
     
        [after the "IBM System/360 Reference Data" card] A summary of
        an assembly language, even if the colour is not green.  Less
        frequently used now because of the decrease in the use of
        assembly language.  "I'll go get my green card so I can check
        the {addressing mode} for that instruction."  Some green cards
        are actually booklets.
     
        The original green card became a yellow card when the
        System/370 was introduced, and later a yellow booklet.  An
        anecdote from IBM refers to a scene that took place in a
        programmers' terminal room at Yorktown in 1978.  A luser
        overheard one of the programmers ask another "Do you have a
        green card?"  The other grunted and passed the first a thick
        yellow booklet.  At this point the luser turned a delicate
        shade of olive and rapidly left the room, never to return.
     
        [{Jargon File}]