Inverting colors in Paint is not something I do frequently. But when you do need this feature occasionally, I have the point of view that Paint is the best tool for such basic image editing. Few days ago, I needed to create a header image for my Youtube channel with a line of inverted colors. I am a Windows power user, yet I was really confused by the user interface of the modern version of MS Paint (I use Windows 8.1).
Starting with Windows 7, the Paint application ships with the Ribbon UI. It looks fancy, but when you need to invert colors of your image, you will not find the appropriate command on any tab of the Ribbon nor in the File menu.