For certain individuals that are visually impaired, inverting the colors of the iPhone and iPad screen may make text easier to read, particularly for users who are colorblind, or partially colorblind. The ability to invert screen colors is a built-in accessibility option that comes with all iPhones and iPads. So whether you yourself are visually impaired or you know someone who is, inverting screen colors may enhance your experience while using iOS.
Full story from the iMore Blog...