Un ejemplo de esto podria ser:
Cita: <script type="text/javascript">
function changeText3(){
var oldHTML = document.getElementById('para').innerHTML;
var newHTML = "<span style='color:#ffffff'>" + oldHTML + "</span>";
document.getElementById('para').innerHTML = newHTML;
}
</script>
<p id='para'>Welcome to the site <b id='boldStuff3'>dude</b> </p>
<input type='button' onclick='changeText3()' value='Change Text'/>
espero te sirva.