VMS Help UIL, Compile Time Errors, ICON_LETTER  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  ERROR:  You have specified a color to be used in an icon that is not
  in  that icon's color table.  The invalid color is identified in the
  message by displaying  the  letter  used  to  represent  that  color
  between  \  \  .  This letter was not defined in the specified color
  table.

  USER ACTION:  Either add the color to the icon's color table or  use
  a  character  representing  a color in the color table.  The default
  color table defines " " as background and "*" as foreground.