$c1 = [ $r, $g, $b ];
$c2 = contrasting_colour( @c1 );

sub contrasting_colour {
   my ( $c ) = @_;
   my ( $r, $g, $b ) = ( 255 - $c->[0], 
                         255 - $c->[1], 
                         255 - $c->[2] );
   return [ $r, $g, $b ];
}

sub rgb2hash {
   my ( $c ) = @_;
   my ( $r, $g, $b ) = ( 255 - $c->[0], 
                         255 - $c->[1], 
                         255 - $c->[2] );
   return sprintf "#%02X%02X%02X",  $r, $g, $b;
}

##</code><code>##

sub contrasting_colour {
   my ( $c ) = @_;
   my ( $r, $g, $b ) = ( ($c->[0] + 128) % 256, 
                         ($c->[1] + 128) % 256, 
                         ($c->[2] + 128) % 256,  );
   return [ $r, $g, $b ];
}