Below is a script for a random color generator using Javascript. An explanation of how it works is in the code comments:

<script>
    function randomColor(){
		return 	"#" +  // start with a leading hash
			Math.random() // generates random number
			.toString(16) // changes that number to base 16 as a string
			.substr(2,6); // gets 6 characters and excludes the leading "0."
	}
</script>

You could then use that function like this:

<div id="test">Some Text Here</div>
<script>
    function randomColor(){
		return 	"#" +  // start with a leading hash
			Math.random() // generates random number
			.toString(16) // changes that number to base 16 as a string
			.substr(2,6); // gets 6 characters and excludes the leading "0."
	}
	document.getElementById('test').style.background = randomColor();
</script>