JSFiddle - React, Tailwind, and code Playground

HTML

<div id="box" style="width: 100px; height: 100px; background-color: brown;"></div>
<select id="selectedColor" onchange="changeColor(this.value)">
  <option value="red">red</option>
  <option value="blue">blue</option>
  <option value="purple">purple</option>
  </select>

JavaScript

window.changeColor = function changeColor(color) {
  document.body.querySelector("#box").style.backgroundColor = color;
}