So I figured out how to achieve the desired GUI behavior. To be clear, I wanted to make the selected tab color the same as the unselected tab background color.
I don't really understand WHY this works, but it does work. Just kinda stumbled upon the solution ;-)
tabbedPane = new JTabbedPane();
If anyone can clarify why this code achieves the affect I wanted, do tell. The UIManager.put does not work alone, as one would think.