ui-select-required.css

.ui-select-bootstrap.ng-invalid span.btn.ui-select-toggle {
  border-color: #D44950 !important;
}

ui-select-required.js

myModule.directive('uiSelectRequired', function () {
    return {
      require: 'ngModel',
      link: function (scope, element, attrs, ngModel) {
        scope.$watch(function () {
          return ngModel.$modelValue;
        }, function (newValue) {
          if (ngModel.$modelValue) {
            ngModel.$setValidity("required", true);
          }
          else {
            ngModel.$setValidity("required", false);
          }
        });
      }
    };
  })