JSFiddle - React, Tailwind, and code Playground

HTML

<DOCTYPE html>
  <html>

  <head>
  </head>

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

  </html>

JavaScript

window.changeColor = function changeColor() {
  var selectColor = document.getElementById("selectedColor").value;
  var box = document.getElementById("box");
  console.log(selectColor)
  box.style.backgroundColor = selectColor;
}