Question: I am accessing my colleague's work email account (on gmail) using IMAP. The problem is that whenever I read one of the messages first, it is marked as read on the server and he often misses it. I would like to keep it unmarked on the server and mark it as read only at my Mail app. 

Is there any way how to setup Mail app so that this behaves like this?

I am accessing his email because he often gets information that is important for me as well and this saves time because he isn't always online and he would have to forward most of it to me anyway.

Answer: 
The mail client in Opera has this feature. I have "Mark as Read" set to "Manually" and email that I have not viewed are shown in blue text and after I viewed them they are shown in black text. And they stay unread.
This is how IMAP works.

Another solution to your problem is to set up Gmail to send you a copy of every message from your co-worker's account.

Otherwise you will have to set each message you read on his account to *Mark As Unread*.