javascript add class to element

HTML

<div id="my_element" class="yay">Hi Im a Element... I think. Wait a minute.. </div>
<button onclick="kx_add_class();">Add Class</button>
<button onclick="kx_add_class_id('my_element','red-col');">Add Red Class</button>

CSS

.red-col {
  color:red;
}
.green-col {
	color:green;
}

Javascript

// you could change this function to pass in the element id and class to add as well, if you want this to be more re-usable
function kx_add_class() {
	document.getElementById("my_element").classList.add("green-col");
}

// ok fine ill add it!
function kx_add_class_id(my_id, my_class) {
	document.getElementById(my_id).classList.add(my_class);
}

// see not that hard to make it re-usable.