CBETA,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.red{
color: #FF0000;
}
.green{
color: #008000;
}
</style>
</head>
<body>
<div id="txt">текст-индикатор</div>
<script>
let answer = prompt("да/нет?", "");
let obj = {"да" : "green", "нет" : "red"};
let cls = obj[answer];
if(cls) txt.classList.add(cls);
</script>
</body>
</html>