Validate Radio Button AngularJS

This seems like it should be fairly easy, but I'm not finding the answer. I have a form where I need to validate that a selection has been made from a radio group. I tried using the 'required' attribute on the radio buttons, but when the form is validated it complains unless all the radio buttons are selected (which is impossible by design).

What is the proper way to validate a radio group selection in AngularJS?

<form name="myForm" ng-submit="submitForm()" ng-controller="ExampleController">
  <input type="radio" ng-model="color" value="red" required>  Red <br/>
  <input type="radio" ng-model="color" value="green" required> Green <br/>
  <input type="radio" ng-model="color" value="blue" required> Blue <br/>
  <tt>color = {{color | json}}</tt><br/>
  <button type="submit">Submit</button>
</form>

Clicking the submit button in the Plnkr shows the behavior.

http://plnkr.co/edit/3qcIbMvJk19OvokcHe2N?p=preview

Try using ng-required="!color". This makes it so that the field is only required when the value is not set. If a value is set, then required is removed and it will pass validation.

<input type="radio" ng-model="color" value="red" ng-required="!color">  Red <br/>
<input type="radio" ng-model="color" value="green" ng-required="!color"> Green <br/>
<input type="radio" ng-model="color" value="blue" ng-required="!color"> Blue <br/>

Here is an updated plunker that demonstrates that the form now validates correctly: http://plnkr.co/edit/EdItU2IIkO1KIsC052Xx?p=preview

Update

e-cloud's answer is simple and doesn't require an additional directive. I suggest everyone use that method if possible. Leaving this answer here as it does provide a working solution and demonstrates the use of the ng-required directive.

I think what you need is to add a name for the radio group, for a radio input need a name to determine which it belongs to, the link below works validation without ng-required(the accepted answer)

<input type="radio" name='group' ng-model="color" value="red" required>  Red <br/>
<input type="radio" name='group' ng-model="color" value="green" required> Green <br/>
<input type="radio" name='group' ng-model="color" value="blue" required> Blue <br/>

http://plnkr.co/edit/LdgAywfC6mu7gL9SxPpC?p=preview