JSFiddle - React, Tailwind, and code Playground

HTML

<div contenteditable="true" id="input"></div>

CSS

div {
    border: 1px solid silver;
}

JavaScript

var input = document.getElementById("input");
input.onkeydown = colorTheText;

function generateRandomColor() {
    var color = [];
    for (var i = 0; i < 3; i++) {
        color.push(Math.floor(Math.random()*250));
    }
    return color;
}

function rgbToHex(color) {
    var hex = [];
    for (var i = 0; i < 3; i++) {
        hex.push(color[i].toString(16));
        if (hex[i].length < 2) { hex[i] = "0" + hex[i]; }
    }
    return "#" + hex[0] + hex[1] + hex[2];
}

function setEndOfContenteditable(contentEditableElement) {
    var range,selection;
    if(document.createRange) {
        range = document.createRange();
        range.selectNodeContents(contentEditableElement);
        range.collapse(false);
        selection = window.getSelection();
        selection.removeAllRanges();
        selection.addRange(range);
    }
}

var colors = [];
var inputLength = 0;

function colorTheText(e) {
    var text = input.innerText;
    if (text.length > inputLength) {
        inputLength++;
        colors.push(generateRandomColor());
    } else {
        inputLength--;
        colors.pop();
        console.log(colors);
    }
    input.innerHTML = "";
    text = text.split("");
    for (var i = 0; i < text.length; i++) {
        if (colors[i]) {
            input.innerHTML += '<span style="color:' + rgbToHex(colors[i]) + '">' + text[i] + '</span>';
        }
    }
    setEndOfContenteditable(input);
}