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;
    # Choose a contrasting colour for text...
    my $fg = contrast_colour($col);
    $btn->configure(
        -bg               => $col,
        -activebackground => $col,
        -fg               => $fg,
        -activeforeground => $fg
    );
}