You could even manage the changes w/o installing software like Designer Client - but that won't help because the suggested solution (by DonAndreas) modifying the Inbox Folder does exactly the same thing like File/Preferences/Mail/Sender Colors (in a much more worse way).
Any modification in design or preferences like this will result in a visible change to all users who are working in this mailbox! All users would see YOUR mails in another color but never their own mails like this.
What you really need is a change where "me" means not only you rather each users name by itself. So Jon Doe should see mails from Jon Doe in another color while Jane Doe sees her own mails in another color but Jon's mails in standard color.
The next problem is the difference beetween sender (from) and sent by (principal) - any previous discussed change would only affect 'from' but never 'principal' - which you need to accomplish your needs.
What could help is a feature called "private on first use" which creates automatically a slightly different (i.e. uses "me" in a dynamic way like @UserName) view for each user. BUT: the Inbox must not exist in multiple copies because the router would only deliver mails to the first one it sees and all others will stay empty. Moving mails between (each users) Inbox and other folders would only affect the users copy of the original Inbox - not visible in the other Inboxes ;) You don't want that 'solution'.
So there is no simple solution as long as we speak about the original Inbox or folders in general.