My Solution

function playAgain() {
    var choice = prompt('Do you want to play again?');
    choice = choice.toLowerCase();
    if ( choice === 'yes' || choice === 'y' ) {
        return true;
    } else {
        return false;
    };
}

function askQuestions() {
    for ( i = 0; i < questions.length; i++ ) {
        var answer = prompt('Q' + (i + 1) + '.' + questions[i][0]);
        answer = answer.toLowerCase();
        if ( answer === questions[i][1] ) {
            questionsCorrect.push(questions[i][0]);
        } else {
            questionsWrong.push(questions[i][0]);
        };
    }
}

function printResults() {
    if (questionsCorrect.length > 0 ) {
        document.write('<h2>You got these questions right</h2><ol>');
        for ( i = 0; i < questionsCorrect.length; i++ ) {
            document.write('<li>' + questionsCorrect[i] + '</li>');
        }
        document.write('</ol>');
    };
    if (questionsWrong.length > 0 ) {
        document.write('<h2>You got these wrong though..</h2><ol>');
        for ( i = 0; i < questionsWrong.length; i++ ) {
            document.write('<li>' + questionsWrong[i] + '</li>');
        }
        document.write('</ol>');
    };
}   

var questions = [
    ['What colour is the sky?', 'blue'],
    ['What colour is the sun?', 'yellow'],
    ['What colour is the grass?', 'green'],
    ['What colour is snow?', 'white'],
    ['What colour is wood?', 'brown']
];

do {
    var correctAnswers = 0;
    var questionsWrong = [];
    var questionsCorrect = [];
    askQuestions();
    document.write("<br><p>Congratulations you got " + questionsCorrect.length + " correct answer(s)!</p>");
    printResults();
} while ( playAgain() )