# given a colour in the form "#ff00ff" i.e. hex rrggbb, return a contrasting colour
sub contrast_colour {
    my $col = shift;
    # Need rgb values for colour...
    if ( not $col =~ /^#(..)(..)(..)$/ ) {
        warn "failed to convert colour '$col' to rgb components\n";
        return;
    }
    my ( $r, $g, $b ) = ( hex($1), hex($2), hex($3) );
    #~ print "RGB= $r, $g, $b\n";
    # here's an idea! simple contrast solution...
    my %oppcolors = (
        "00" => "FF",
        "33" => "FF",
        "66" => "FF",
        "99" => "FF",
        "CC" => "00",
        "FF" => "00",
    );
    $r = int( $r / 51 ) * 51;
    $g = int( $g / 51 ) * 51;
    $b = int( $b / 51 ) * 51;
    #~ print "RGB rounded = $r, $g, $b\n";
    $r = $oppcolors{ sprintf( "%02X", $r ) };
    $g = $oppcolors{ sprintf( "%02X", $g ) };
    $b = $oppcolors{ sprintf( "%02X", $b ) };
    #~ print "RGB inverted = $r, $g, $b\n";
    return "#$r$g$b";
}