WordNet (r) 3.0 (2006):

green card
    n 1: 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"

The Jargon File (version 4.4.7, 29 Dec 2003):

green card
 n.

    [after the IBM System/360 Reference Data card] A summary of an assembly
    language, even if the color is not green and not a card. 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.?

    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.

    In fall 2000 it was reported from Electronic Data Systems that the green
    card for 370 machines has been a blue-green booklet since 1989.


The Free On-line Dictionary of Computing (30 December 2018):

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]