Originally Posted by HarryT
With the very greatest respect, I've answered this question at least half a dozen times already. It's because the colour fringing is due to very slight non-parallelism in the two faces of the light diffuser. These are difference of the order of the wavelength of light - ie way, way below what any reasonable manufacturing tolerance can achieve.
If the technology of a reasonable manufacturing tolerance could not be obtained then the Paperwhite should not have been mass produced and released. Perhaps the technology should of been abandoned for something achievable IMO.