title: Change button color after click in Angularjs
body: <p>This is what I have now:
  index.html</p>
  
  <pre><code>&lt;div ng-app&gt;
    &lt;h2&gt;Todo&lt;/h2&gt;
    &lt;div ng-controller="TodoCtrl"&gt;
      &lt;button class="btn btn-danger" ng-click="start()"&gt;Start&lt;/button&gt;
    &lt;/div&gt;
  &lt;/div&gt;
  
  
  function TodoCtrl($scope) {
    $scope.start = function() {
  
    };
  }
  </code></pre>
  
  <p>What can I fill in the <code>$scope.start</code> function so that once the button is clicked, the color of the button turns yellow.</p>