PENCIL /COLOUR

        PENCIL [N] /COLOUR Arg

    Modify the current pen color, or the color of the N-th pen.

    The argument can be a color name (subset of  the  X11  colors  catalog),
    e.g. RED, LIGHT_SEA_GREEN, MEDIUM_BLUE... The complete list is available
    by typing "PENCIL /COLOUR ?". The colors can also be displayed thanks to
    the  demonstration  procedure "@ gag_demo:demo-pencolors.greg". The list
    is also available in the character array GTV%COLORS for  use  in  proce-
    dures.

    There are 3 special color names:
      BACKGROUND  resolves  as WHITE on white devices and BLACK on black de-
                 vices,
      FOREGROUND resolves as BLACK on white devices and WHITE on  black  de-
                 vices,
      NEGATIVE  inverts  the  pixel colors below the graphical segment begin
                 drawn. This ensures that the  segment  is  always  visible,
                 whatever  the context. NEGATIVE is supported only for X and
                 PNG devices. Other devices use the FOREGROUND  color  as  a
                 fallback.

    The argument can also be an integer value (explicit, or a variable name,
    or the result of a math expression) from -1 to  23  with  the  following
    signification:
      -1 NEGATIVE
       0 FOREGROUND
       1 RED
       2 GREEN
       3 BLUE
       4 CYAN
       5 YELLOW
       6 MAGENTA
       7 BACKGROUND
       8  to 23: those 16 colors can be customized by the user (see HELP LUT
         /PEN for details). They can be used as any other  color,  but  they
         also correspond as the "fill pens" used by the command RGMAP /GREY.
         Those colors are dynamic, i.e., by default, changing them will also
         modify  the  segments  previously  drawn with them (see HELP CHANGE
         LUT).