Sometimes you may have a button (or other control) that can have a different appearance depending on the different focus statuses.
For example, the default status without focus is a grey image, whereas when the mouse hovering over it becomes blue. Then when the mouse left button is held down on the button (and not released), the image changes to a red image.
Finally, when the button is activated using the tab key to tab into the button and the mouse is somewhere else, it has a fourth color.
This article discusses solutions for how to test these kinds of situation