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.