
It is hard to believe that Excel still doesn't have the built-in ability to display a cell split diagonally with independent labels in each half. There are several ways to solve this manually but with issues. The two main issues are that resizing cells can mess up some solutions and most solutions do not allow you to color each half independently.
This article demonstrates two complete solutions - one is an add-in that I built and the other is a manual process to do the same thing.