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: 
This mechanism is a feature of the mail protocol that you are using, and isn't a local feature that you can choose to set within Mail. You are likely useing either IMAP or Exchange, the the replication of mail states for things like location, unread/read etc is a part of the normal operation.

I am not at a Mac, so I cannot test for features, but the best you can hope for is a configuration in Mail that will allow you to either change what action marks the mail as read or not (Might be mere selection, might be opening the time specifically, might be viewing the message for more than X seconds etc) or extend the time that it takes for the action to complete (viewd for 5 seconds, or 10, etc).

You can of course simply right click on a read mail and then mark it as unread again.
I had same issue, the culprit was the iPhone's email client with its default preview feature. A preview fools the IMAP to believe it's read. Solution: to switch it off (go to settings -> email -> set preview to none.