There’s only a few tag-cloud plugins that still work – most of them have stopped being supported.
The best one I found has super-awesome-multi-colour mode. But by default it’s disabled, and the config-options don’t include a way to turn it on. You have to dig in the developer documentation to find out how.
ALSO … by default, it uses your theme’s CSS styles for the cloud. That is a really bad idea – it disables most of the tag-cloud features (including the “cloud” part!). Again, unfixable unless you dig in the dev docs to find the secret command.
For future reference (because I’m pretty sure I’ll be using this on other sites soon), here’s a working PHP snippet you can embed in your Template source (I hate that WordPress still has this stupid bug where you can only modify pages by writing new PHP files – so stupid and unnecessary!) in your Theme folder:
$args = array( “title” => “”, “color” => “random”, “text_transform” => “capitalize”, “link_italic” => “yes”, “hover_underline” => “yes”, “alignment” => “center”, “link_bg_color” => “#DDDDDD”, “separator” => ” – “, “show_title_text” => 0, “avoid_theme_styling” => “true”, “max” => 50 );
do_utcw( $args );