PENCIL COLORS

        There  are 25 colors available for PENS. Colors -1, 0 and 7 are SPE-
    CIAL, colors 1 to 6 are FIXED (named colors or their equivalent shade of
    grey on grayscale terminals). The last 16 are user-defined.

    Color  0  (the  default  for the default PEN 0) is ALWAYS the foreground
    color (i.e., always visible whatever the background color of the graphic
    window  is),  and becomes BLACK on the paper hardcopy. Color 7 is ALWAYS
    the background color, useful for text written on an grey-scale image for
    example.

    Colors  1 to 6 are defined as follows for a standard display with suffi-
    cient colors available in its colormap:
        "red","green","blue","cyan","yellow" and "magenta"
    For grey-scale displays, it becomes:
        "grey42","grey70","grey14","grey28","grey56" and "grey84"
    This may vary depending on the number of available colors on  your  dis-
    play.

    The  last  16 colors are user-definable (see HELP LUT) and correspond to
    the 16 'FILL PENS'  used  as  fill  colors  in  various  commands,  like
    RGMAP /GREY or POLYGON /FILL.

    Finally,  the  color -1 is negative, which means it will have an inverse
    color of what is behind. It is useful to draw segments which are  always
    visible,  whatever the context. This special color is available only un-
    der X and PNG devices. For the others, the foreground color (0) is used.