well, how you describe it, it isnt actually possible via extension, it would be a usefull feature though (I wouldnt say its worth extra realestate bar though)
My Highlight tabs idea, would function in the same way this does.
For my idea, you can choose the background color of any tab/tabs.
A tabs manager would be a great addition to chrome (I currently use "TabSearch" as it barely uses any memory, as opposed to some of the similar programs that show thumbnails.
In tabs, task manager, you would be able to highlight multiple tabs at once, search tabs (not just the tab name+url, but a checkbox for content), arrange in certain patterns (last viewed, Group by Url, ect)
along with options to preform on selected tabs (highlight, close, bookmark, ect)