I love that Zimbra has the ability to use tags, but I think the way they're displayed in the conversation/message list is a little off.
The colors are ok, but since there are so few easily distinguished colors, you end up with tags looking pretty similar. I think the gmail way of tagging in the subject line is superior, especially if a message has multiple tags.
An example subject with multiple tags:
[Exchange][Hosting] I'd like to cancel my account
[Zimbra][Hosting] I need a new account
This is especially important for multiple tags, but even in the case where there is a single tag, the subject text tagging gives you a better idea of how amessage is tagged without having to open the message.
I realize that you can mouseover, but if you have 100s of messages in your list, it's nice to have the text out there.
just a thought.