Despite its name, the Invert effect doesn’t turn the display upside-down. Rather than the image itself, it is the color values in the image that are inverted: each pixel is redrawn in its complementary light intensity and/or color, producing a readily recognizable but recolored image.
This effect uses the YCrCb color model, which has one channel for luminance (brightness information) and two channels for chrominance (color information). The YCrCb model is often used in digital video applications.
Original image (top left) and the seven colorings available with the Invert effect.