Main Menu: Image > Adjust > Invert

Inverts the colors in an image or area. You may use this command to make a positive black-and-white image negative or to make a positive from a scanned black-and-white negative. When you invert an image, the brightness value of each pixel in each image channel is converted to the inverse value on the 256-step color-values scale. For example, a pixel in a positive image with a value of 255 is changed to 0, and a pixel with a value of 10 to 245.