Because you passed $colour by value, as opposed to $rightwrong, which was passed by reference. In the former case the method gets a copy of the state the variable had at invocation time. Obviously then you can change the variable all you like - it won't change the copy. To do what you wanted, you need to reconfigure the widget.

# ...

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');
    }

# ...
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(If I am allowed a personal comment, I really hate Tk's way of using pass-by-reference to keep track of changes. It leads to spaghetti code real quick like.)

Makeshifts last the longest.