JSFiddle - React, Tailwind, and code Playground

HTML

<span id="test">abcdfdfdfdfdfdfd</span>

JavaScript

var str = $('#test').html();
function color(noOfCharacters){
$('#test').html('<span style="color:red">'+str.substr(0,noOfCharacters)+'</span><span>'+str.substr(noOfCharacters,str.length)+'</span>');
}
color(4);