<div id="ab">aaaaaaaaa</div>
<script>
var ac = document.getElementById("ab").innerHTML.substr(0,1);
ac.style.color='red';
</script>
<script>
var ac = document.getElementById("ab").innerHTML.substr(0,1);
ac.style.color='red';
</script>
<div id="ab">abcdef</div>
<script>
var o=document.getElementById("ab")
var ac = o.innerHTML.substr(0,1);
ac="<span style='color:red'>"+ac+"</span>"+o.innerHTML.substr(1);
o.innerHTML=ac;
</script>