OK, I've tidied up my first draft and added a little Tk testing toy. The contrasting colours returned tend toward the lighter but the hash is easily tweaked.

#! perl
use strict;
use warnings;
use Tk;
my $title = "colour contrast";
my $mw    = new MainWindow( -title => $title );
my $fr    = $mw->Frame()->pack;
for ( 1 .. 5 ) {
    my $b = $fr->Button( -text => "Colour $_", )->pack;
    $b->configure( -command => [ \&pick_n_mix, $b ] );
}
MainLoop();

sub pick_n_mix {
    my ($btn) = @_;
    my $old = $btn->cget( -bg );
    my $col = $mw->chooseColor(
        -title        => "Choose a nice colour...",
        -initialcolor => $old
    );
    return unless $col;
    $btn->configure( -bg => $col, -activebackground => $col );
    # Choose some contrasting colours...
    my $fg = contrast_colour($col);
    $btn->configure(
        -bg               => $col,
        -activebackground => $col,
        -fg               => $fg,
        -activeforeground => $fg
    );
}
# 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 $col =~ /^#(\w\w)(\w\w)(\w\w)$/ ) {
        warn "failed to convert colour '$col' to rgb components\n";
        return;
    }
    my ( $r, $g, $b ) = map hex, ( $1, $2, $3 );
    # Use a hash of contrasting values for the "websafe pallette"
    # with emphasis on lighter colours...
    my %opp = qw(00 FF 33 FF 66 FF 99 FF CC 00 FF 00 );
    # Round each value down to a multiple of hex 33 (i.e. 51)...
    map { $_ = int( $_ / 51 ) * 51 } ( $r, $g, $b );
    # Replace each value with a contrasting value from the hash...
    map { $_ = $opp{ sprintf "%02X", $_ } } ( $r, $g, $b );
    return "#$r$g$b";
}
[download]