Apple issues support document related to "purple halo" on Apple iPhone 5 pictures
Camera Image Effects," this problem happens with all small cameras and even with the camera on each and every generation of the Apple iPhone. The document calls the purple coloring on the photos a "spot" or "haze" or "flare" that occurs on the edge of the frame. According to Cupertino, when the light source is out of the viewfinder, it can reflect on the camera sensor causing the purple tint.
Apple suggests that if you move the camera slightly to change the angle at which the light source enters the camera, or shield the camera lens with your hand, it will "minimize or eliminate the effect." Hopefully, either of these suggestions will be able to allow Apple iPhone 5 users to take pictures free of that annoying purple tint, halo, spot or whatever you want to call it.
source: Apple via SlashGear