<head>
<script src = "http://code.jquery.com/jquery.min.js"></script>
<style>
#btn {
background: red ;
}
</style>
</head>
<body>
<textarea id = "name"></textarea>
<textarea id = "email"></textarea>
<textarea id = "msg"></textarea>
<button id = "btn">GO</button>
<script>
var textareas = $("#name,#email,#msg"),
button = $("#btn") ;
textareas.on("change input", function() {
var color = "green" ;
textareas.each(function() {
if(!$(this).val())
color = "red" ;
}) ;
button.css("background", color) ;
}) ;
</script>
</body>