quick test

by thebabydino

HTML

<div class='boo'>boo</div>

CSS

.boo { background: lightblue; -webkit-transition: 1s; }
.purpleme { background: purple; }

JavaScript

var boo = document.querySelector('.boo');

boo.addEventListener('click', function(){
  if(!this.classList.contains('purpleme'))
    this.classList.add('purpleme');
  else
    this.classList.remove('purpleme');
}, false);