JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li>test</li><li>test</li>
    <li>test</li><li>test</li>
</ul>

CSS

ul li:nth-child(4n+1),
    ul li:nth-child(4n+2) {
      background: #CCC;
    }
    ul li:nth-child(4n+3),
    ul li:nth-child(4n+4) {
      background: #DDD;
    }