How to change text color in different fragments

You could simply do

<section>
  <h1>Title</h1>
  <p class="fragment highlight-red" data-fragment-index="1" >Text</p>
  <p class="fragment data-fragment-index="1"">Fragment</p>
</section>

That way, the red text and the "Fragment" text will always be in sync.

It's not rather simple:

<section>
  <h1>Title</h1>
  <p id="postfragment">Text</p>
  <p class="fragment">Fragment</p>
</section>
...
<script>
Reveal.addEventListener('fragmentshown', function(event) {
   document.getElementById("postfragment").style.color="red"; 
});
</script>