The Free On-line Dictionary of Computing (19 January 2023):

green bytes
green words

    (Or "green words") Meta-information embedded in a
   file, such as the length of the file or its name; as opposed
   to keeping such information in a separate description file or
   record.

   By extension, the non-data bits in any self-describing format.
   "A GIF file contains, among other things, green bytes
   describing the packing method for the image".

   At a meeting of the SHARE Systems Division, November 22, 1964,
   in Washington, DC, George Mealy of IBM described the new
   block tape format for FORTRAN in which unformatted binary
   records had a Control Word.  George used green chalk to
   describe it.  No one liked the contents of the Green Word (not
   information, wrong location, etc.) so Conrad Weisert and
   Channing Jackson made badges saying "Stamp out Green Words".
   This was the first computer badge.

   Compare out-of-band, zigamorph, fence.

   Button 251 (http://mxg.com/thebuttonman/search.asp).

   [Jargon File]

   (1994-11-02)


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

green bytes
 n.

    (also green words)

    1. Meta-information embedded in a file, such as the length of the file or
    its name; as opposed to keeping such information in a separate description
    file or record. The term comes from an IBM user's group meeting (ca. 1962)
    at which these two approaches were being debated and the diagram of the
    file on the blackboard had the green bytes drawn in green.

    2. By extension, the non-data bits in any self-describing format. “A GIF
    file contains, among other things, green bytes describing the packing
    method for the image.” Compare out-of-band, zigamorph, fence (sense
    1).