random color for a button

I made my own personal startup page for my browser with some of my preferred sites and I wanted to make something interesting. My idea was 'random' button that kept changing color and also always had a random link. I am having trouble getting it to work.

  //CSS
  animebutton{
  background-color: #1d439b;
  border-radius: 25px;
  border: none;
  color: white;
  padding: 15px 32px;
  text-align: center;
  text-decoration: none;
  display: inline-block;
  font-size: 10px;}

//HTML
<animebutton id='animebutton' onload="randomcolor();"><a href="http://ift.tt/2gXyNwl">MyAnimelist</a></animebutton>

 //JAVAASCRIPT
 <script type="text/javascript">
  function randomcolor(){
    var color = '#';
    var letters = ['000000','FF0000','00FF00','0000FF','FFFF00','00FFFF','FF00FF','C0C0C0'];
    color += letters[Math.floor(Math.random() * letters.length)];
    document.getElementbyId('animebutton').style.backgroundColor = color;
}