[рони]

<style>
  .red {
    color: red;
  }
</style>
<p class="hello">Lorem ipsum dolor sit amet, consectetur</p>
<p class="hello">Sapiente in delectus dolorem reiciendis ditii</p>
<p class="hello">Quis dolore autem voluptatibus repellat</p>
<script>
      var p,
      timer = setInterval(function () {
      p = document.querySelector('p.hello:not(.red)');
      p ?  p.classList.add('red') : clearInterval(timer)
    }, 2000);
</script>