Answer:
The action Invert on any given Photoshop version makes the colors on the selected layer(s) become their opposites on the color scale. For example, black will become white, and visa versa. Dark gray will become Light Gray, and visa versa. The only exception to this is when you have a shape or part of a shape which is a shade of gray exactly 50% black and 50% white, in which case there will be no color change.