When browsing your keitai display, you usually look perpendicularly from a 90 degree angle, straight on. However, if you look from about a 15 degree angle, the screens colors will invert. Looking at an inverted picture this way will, of course, turn the colors back to normal.
This reminds me of a favorite tecnique of inverting pictures in high school photoshop class. After exposing a photo and washing it in the developing solution, if you once again expose it with plain light for a short period of time, it will create an image similar to the negative.