JSFiddle - React, Tailwind, and code Playground

HTML

<p>Here is a dandy block of text to color up</p>
<p> WHOAH another paragraph</p>

JavaScript

var p_index = 0,
    char_start = 10,
    char_end = 21;

$('p').eq(p_index).html(function(i,currentText) {
     return currentText.substring(0, char_start) +
            "<span style=\"background:yellow\">" +
            currentText.substring(char_start, char_end) +
            "</span>" +
            currentText.substring(char_end);
});