The back of the reader is a metal sheet.
Theoretically they should reflect more light, when light is shone upon them.
A white background, will still look a bit grey in a darker environment, and will not reflect as much light as a metal background. Plus, when reading it in sunlight, the white might be too bright for your eyes!
I would also believe that the metallic substrate is part of the LCD display. Removing it would probably damage the screen. But I've never opened it to verify.
Know, that because of the LCD technology, you'll never have as bright of a background as paper, or even e-ink, even if you would be able to do the mod!