Che ne pensi di una cosa cosi?

codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"> 
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" /> 
<title>Documento senza titolo</title>
<style>
.block
{
color: white;
width: 150px;
height: 70px;
margin: 10px;
}
.back1
{
background:lightgreen;
}
.back2
{
background:lightblue;
}
</style>
</head>
<body>
<div class="block back1" onclick="this.className='block back2'">
Clicca qui!
</div>
</body>
</html>