I believe the issue is not in the shade of white but in the degree of translucency of the coating. On all Apple phones up to now the black frame around the screen (or at least parts of it) lets light through for the proper functioning of the ambient light and proximity sensors. At the same time these sensors are invisible for the user - it's a flat black frame. The iPhone 4 is the first one with white front and it seem that Apple and the manufacturer of the screen glass are having hard time achieving the same effect.