Eric Scheid (founder of the excellent IAWiki) shot me a line with a more efficient and much sleeker version of my random hex code straight after I posted it, so here it is!

function random_hex() {
	$color = '';
	for ($c = 0; $c < 6; $c++) {
		$i = rand(0, 15);
		$color .= substr("0123456789ABCDEF", $i, 1)
	}
	$color = '#' . $color;
	return $color;
}

Thanks Eric!