Few things, its not line 443 of GD.pm, its line 443 of Graph.PM. And from the GD::Graph POD...

=item dclrs (short for datacolours)

This controls the colors for the bars, lines, markers, or pie slices.
This should be a reference to an array of colour names as defined in
L<GD::Graph::colour> (S<C<perldoc GD::Graph::colour>> for the names av
+ailable).

    $graph->set( dclrs => [ qw(green pink blue cyan) ] );
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This makes your color choices fail because later on in Graph you there is a line (at line # 443) that does a:

    _rgb($self->{dclrs}[$_[0] % @{$self->{dclrs}} - 1]);
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Update:

If you want to specifiy colors outside of the redefined list in GD::Graph::colours, you must use:

add_colour(colourname => [$r, $g, $b]) or add_colour('#7fe310')

Self-explanatory. Exported with the :colours tag.
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