If you don't want to mess with the umlauts, you can also use Koeln. That spelling may be more common on the web. Here is a link to the Koelner Dom web site: http://www.koelner-dom.de/ . Click on the English flag at the top for the English language version.
But I do notice the use of the substitute 'e' more on the internet. It just looks funny to me, though.