By using e-rte-placeholder class, you can customize the placeholder style.
var defaultRTE = new ej.richtexteditor.RichTextEditor({
placeholder:'Type Something',
});
defaultRTE.appendTo('#defaultRTE');<!DOCTYPE html><html lang="en"><head>
<title>Essential JS 2 Rich Text Editor</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="description" content="Typescript UI Controls">
<meta name="author" content="Syncfusion">
<link href="index.css" rel="stylesheet">
<link href="//cdn.syncfusion.com/ej2/21.1.35/ej2-base/styles/material.css" rel="stylesheet">
<link href="//cdn.syncfusion.com/ej2/21.1.35/ej2-richtexteditor/styles/material.css" rel="stylesheet">
<link href="//cdn.syncfusion.com/ej2/21.1.35/ej2-inputs/styles/material.css" rel="stylesheet">
<link href="//cdn.syncfusion.com/ej2/21.1.35/ej2-lists/styles/material.css" rel="stylesheet">
<link href="//cdn.syncfusion.com/ej2/21.1.35/ej2-navigations/styles/material.css" rel="stylesheet">
<link href="//cdn.syncfusion.com/ej2/21.1.35/ej2-popups/styles/material.css" rel="stylesheet">
<link href="//cdn.syncfusion.com/ej2/21.1.35/ej2-buttons/styles/material.css" rel="stylesheet">
<link href="//cdn.syncfusion.com/ej2/21.1.35/ej2-splitbuttons/styles/material.css" rel="stylesheet">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
<script src="https://cdn.syncfusion.com/ej2/21.1.35/dist/ej2.min.js" type="text/javascript"></script>
</head>
<body>
<div id="container">
<script src="https://cdn.jsdelivr.net/npm/marked/marked.min.js"></script>
<div id="defaultRTE">
</div>
</div>
<style>
.e-richtexteditor .e-rte-placeholder {
/* placeholder style */
font-family: monospace;
color: deeppink;
}
</style>
<script>
var ele = document.getElementById('container');
if(ele) {
ele.style.visibility = "visible";
}
</script>
<script src="index.js" type="text/javascript"></script>
</body></html>