[Dilettante_Pro]

<input type="button" value="Первая">
<input type="button" value="Вторая">
<input type="button" value="Третья">
<script>
 var btns = document.querySelectorAll('input[type=button]');
for(var i = 0; i < btns.length; i++) {
    btns[i].onclick = function(){
       for(var j = 0; j < btns.length; j++) {
           btns[j]==this? btns[j].style.color = 'red': btns[j].style.color = 'green'; 
      }
   }
}
</script>