When it comes to making presentation changes, it's best practice to handle them within the view rather than the controller.
ng-style gives you the flexibility to assign AngularJS class objects that can be dynamically altered.
Take a look at this example snippet from the documentation:
<input type="button" value="set color" ng-click="myStyle={color:'red'}">
<input type="button" value="set background" ng-click="myStyle={'background-color':'blue'}">
<input type="button" value="clear" ng-click="myStyle={}">
<br/>
<span ng-style="myStyle">Sample Text</span>
<pre>myStyle={{myStyle}}</pre>
Edit:
If you need to conditionally apply ng-style based on function calls in your controller, here's how you can do it:
<td ng-style="{ background-color: clicked ? 'red' : 'black' }">
In your controller function:
var $scope.clicked = false;
var ChangeColor = function($index){ $scope.clicked = true; }
Remember, you can also conditionally apply pre-defined style classes using ng-class (check out this related question for examples).