color_index
Change the graphics pen colour.

PARAMETERS
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color_index   -   The PGPLOT color index for subsequent graphics.
                    0 - Background (black).
                    1 - White.
                    2 - Red.
                    3 - Green.
                    4 - Blue.
                    5 - Cyan.
                    6 - Purple.
                    7 - Yellow.
                    8 - Orange.
                    9 - Green-yellow (chartreuse).
                   10 - Spring green.
                   11 - Light blue.
                   12 - Purplish blue.
                   13 - Magenta.
                   14 - Dark grey.
                   15 - Light grey.