JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">

  <head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Example</title>
  </head>

  <body>
    <button id='btnAddClass'>Add class</button>
    <button id='btnRemoveClass'>Remove class</button>
  </body>

</html>

CSS

.blue {
  color: blue;
}

JavaScript

const removeActiveClass = () => {
  const activeElements = document.getElementsByClassName('blue');
  for (const activeElement of activeElements) {
    activeElement.classList.remove('blue');
  }
};

const onClick = (e) => {
  removeActiveClass();
  e.target.classList.add('blue');
};

document.getElementById('btnAddClass').addEventListener('click', onClick);
document.getElementById('btnRemoveClass').addEventListener('click', onClick);