JSFiddle - React, Tailwind, and code Playground

HTML

<input type='button' value='Change color' />

<div id='para'>
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</div>

JavaScript

$('input').click(function(){

    var para = $('#para');
    var charArray = $('span', para);
    
    // case: No span (Initial String)
    if (charArray.length === 0) {
        var html = para.html();
        var newArr = [];
        var len = html.length;
        for (var i=0; i<len; i++) {
            newArr.push('<span>' + html[i] + '</span>');
        }
        html = newArr.join('');
        para.html(html);
        charArray = $('span', para);
    }
    
    // Reset all spans
    $.each(charArray, function(i, value) {
        value.style.color = '';
    });
    
    var paralen = charArray.length;
    
    for (var i=0; i<50; i++) {
        var pos = Math.floor(Math.random()*paralen);
        charArray[pos].style.color = 'red';
    }

});