<?php wp_tag_cloud(); ?>

<?php wp_tag_cloud('number=50&largest=24&smallest=12&unit=px'); ?>

function colorCloud($text) {
$text = preg_replace_callback('|<a (.+?)>|i', 'colorCloudCallback', $text);
return $text;
}
function colorCloudCallback($matches) {
$text = $matches[1];
$color = dechex(rand(0,16777215));
$pattern = '/style=('|")(.*)('|")/i';
$text = preg_replace($pattern, "style="color:#{$color};$2;"", $text);
return "<a $text>";
}
add_filter('wp_tag_cloud', 'colorCloud', 1);