input { background:red }
 input:indeterminate { background:green; }

 <div>

  <script>
   

    function test() {
      /* Technically not part of DOM1 HTML but Tantek assures me that
      is an oversight... */
      document.getElementById("test").indeterminate = true;
    }

    window.setTimeout("test()", 100);
   
  </script>

  <p> <input id="test" type="checkbox"> This checkbox should have a green background. </p>

 </div>