To access this command, click on "Colors -> Invert" on the user menu.
The "Invert" command inverts colors in an image. When you apply the "Invert" command to an image, the brightness value for each pixel in an image will be converted to the inverse value in the scale consisting of 256 colors. For example, for a certain pixel, a value of 255 will be converted to 0, and for another pixel a value of 5 will be changed to 250.