$ vi app/assets/templates/users/new.html.erb

<div ng-controller="UsersNewCtrl" class="row">
<style>
  form.ng-invalid.ng-dirty { background-color: lightpink; }
  form.ng-valid.ng-dirty { background-color: lightgreen; }
  form { padding: 10px;}
</style>
  <div class="col-md-6 col-md-offset-3">
    <h1 class="text-center">Sign up</h1>
    <form name="userNewForm" novalidate>
      <div class="well">
        <div class="form-group" ng-class="{'has-error': userNewForm.name.$invalid && userNewForm.name.$dirty}">
          <label>Name</label>
          <input name="name" class="form-control"
            ng-model="user.name"
            required
            ng-maxlength="20" />
          <span class="text-danger" ng-show="userNewForm.name.$dirty && userNewForm.name.$error.required">
            Name can't be blank
          </span>
          <span class="text-danger" ng-show="userNewForm.name.$dirty && userNewForm.name.$error.maxlength">
            Name is too long (maximum is 20 characters)
          </span>
        </div>
        <div class="form-group" ng-class="{'has-error': userNewForm.email.$invalid && userNewForm.email.$dirty}">
          <label>Email</label>
          <input type="email" name="email" class="form-control"
            ng-model="user.email"
            required />
          <span class="text-danger" ng-show="userNewForm.email.$dirty && userNewForm.email.$error.required">
            Email can't be blank
          </span>
          <span class="text-danger" ng-show="userNewForm.email.$dirty && userNewForm.email.$error.email">
            Email is invalid
          </span>
        </div>
        <div class="form-group" ng-class="{'has-error': userNewForm.password.$invalid && userNewForm.password.$dirty}">
          <label>Password</label>
          <input type="password" name="password" class="form-control"
            ng-model="user.password"
            required
            ng-minlength="6" />
          <span class="text-danger" ng-show="userNewForm.password.$dirty && userNewForm.password.$error.required">
            Password can't be blank
          </span>
          <span class="text-danger" ng-show="userNewForm.password.$dirty && userNewForm.password.$error.maxlength">
            Password is too short (minimum is 6 characters)
          </span>
        </div>
..............
        <button ng-click="submit()" class="btn btn-primary"
                ng-disabled="userNewForm.$invalid" >
          Create my account
        </button>
      </div>
    </form>
  </div>
</div>