JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
  <li class="parent">1.</li>
  <li class="child">1. first</li>
  <li class="child">1. second</li>
  <li class="child">1. third</li>

  <li class="parent">2.</li>
  <li class="child">2. first</li>
  <li class="child">2. second</li>
  <li class="child">2. third</li>

  <li class="parent">3.</li>
  <li class="child">3. first</li>
  <li class="child">3. second</li>
  <li class="child">3. third</li>

  <li class="parent">4.</li>
  <li class="child">4. first</li>
  <li class="child">4. second</li>
  <li class="child">4. third</li>
</ul>

CSS

li:nth-child(2n) { background: red; }
li:nth-child(4n) { background: whitesmoke; }