If I wanted to use colour to identify unprocessed emails, I would:
Go to View -> Change View -> <Current view settings> -> Modify -> Columns. I would then add "Categories" to the visible columns.
I would use the AddItem event to set the Category for all new emails to red.
- I would use the MailItem Reply and Forward events to the set the Category to green.
This would give a block of red or green against every email in Inbox.
However, I would not use colour. I would use the MailItem Reply and Forward events to move the item to a different folder named something like "Processed". With this approach, only the unprocessed emails would remain in Inbox.