I'm assuming you've got developer level access to the database, are aware of the ramifications of changing the structure of your database vs changing the template, and always follow safe practices when changing default settings (i.e. BACK UP YOUR FILE FIRST!!!!). If all that's so, go into Design, Folders and open *($Inbox). Right-click the top pane and choose "Folder Properties..." from the context menu. Go to the colored "S" tab and you'll see several pull-downs. The one you want is "Unread rows" Select your color, then save and close.
(CMA: Though this is a simple enough change, there's alot of other things you can get into here that'll totally muck up your database if you're not careful, and I won't be held responsible if that happens.)