# ...

my $rw = $mw->Label(-textvariable => \$rightwrong,
           -foreground => $colour)->pack(-side=> 'bottom');

# ...

    if ($answer == $guess) {
        #count correct answers
        $correct++;
        $rightwrong = 'Correct!';
        $rw->configure(-foreground => 'green');
    }else{
        $rightwrong = 'Wrong!';
        $rw->configure(-foreground => 'red');
    }

# ...