Barney Britton and Kelcey Smith for Digital Photography Review:

The most likely cause of the iPhone 5’s purple haze is probably lens flare and internal reflections in the camera lens assembly. All lenses are succeptable to lens flare to some degree, and as you can see from the images at the top of this page, the iPhone 4S isn’t immune either (ditto the iPhone 4 and competitive smartphones from other manufacturers).

It’s a shame that this even has to be said, but apparently many tech writers are so profoundly stupid, they don’t understand it.