Alexprom,
<!doctype html>
<html>
<head>
<title></title>
<meta charset='utf-8' />
<style>
.red {
color: Red;
}
.green {
color: Green;
}
.blue {
color: Blue;
}
</style>
</head>
<body>
<select name="sel" id="sel">
<option value="">-- Выбрать --</option>
<option value="red">red</option>
<option value="green">green</option>
<option value="blue">blue</option>
</select>
<div id="toShow" class="variations_button bl">Hello, I am hidden block</div>
<script>
var sel = document.getElementById('sel'),
div = document.getElementById('toShow'),
current;
sel.addEventListener('change', function(event) {
if (current) div.classList.remove(current)
if (this.value) {
current = this.value;
document.getElementById('toShow').classList.add(current);
}
})
</script>
</body>
</html>