JSFiddle - React, Tailwind, and code Playground

HTML

<div class="table">
  <div class="red"></div>
  <div class="blue"></div>
  <div class="black"></div>
</div>
<div id="but">Click</div>

CSS

.table {
  background: #ccc;
  text-align: center;
}

.table div {
  display: inline-block;
  width: 50px;
  height: 50px;
  margin: 15px;
}

.red {background: red}
.blue {background: blue}
.black {background: black}

JavaScript

but.onclick = function(){
	//add class
}