Suppose in your HTML, you have a paragraph as follows
Sentence
You can do this in javascript. Wrap each letter in a tag.
var par = document.getElementById("myPar");
var colors = ["red", "blue", "green", "orange"];
par.innerHTML = par.innerHTML.split('').map(function(c) {
return "" + c + ""
}).join('');
http://jsfiddle.net/ua9of1ux/1/