try this to get the box-shadow under your full control.
<html>
<head>
<style>
div {
width:300px;
height:100px;
background-color:yellow;
box-shadow: 0 10px black inset,0 -10px red inset, -10px 0 blue inset, 10px 0 green inset;
}
</style>
</head>
<body>
<div>
</div>
</body>
</html>
this would apply to outer box-shadow as well.