ng-class expression

HTML

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.6/angular.min.js"></script>
    <div ng-app>
        <a ng-click="flags.open=!flags.open">Click me<div ng-class="{'active':flags.open}" class="isRed">
            Class "active" is present: {{flags.open}}
    </div>

CSS

.active {
    color: red;
}

.isRed{
  background:green;
}