Share Color Map Entries

  This option is enabled on single-plane and true color systems
  and disabled on GPX systems.  Clicking on this option
  prevents DECterm from allocating color map entries for
  its exclusive use.  Instead, DECterm allocates shared, read-
  only colors from the default color map.  This means not only
  that DECterm does not need to create a private color map
  which would prevent the windows from displaying in their
  true colors, but also that DECterm colors may not match the
  colors used on a terminal.