From the Jargon File
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