From ZuneInsider :

We have maintained the double-shot plastic for this year’s devices. The surprise this year is that we used a light inside color and a tinted outside shot that carries the hue.  The effect of the new double-shot: darkening the edges instead of lightening them (like last year’s treatment).  We think this not only creates an interesting colored bottle effect, but it also give the colors more range.  So depending on your lighting situation, you will see the colors move to being more or less vibrant.  This effect is most obvious with the red unit between inside and outside lighting conditions. 

We also decided to use a metal back, so as to ensure we reduce the thickness of the overall device, making it a bit easier to carry around.  This effect, along with the matte finish, extends our line of low maintenance, smudge resistant products. However, with the Red Zune 4GB and Zune 8 GB, we made an exception and went with a glossy finish. These, quite frankly, needed to be glossy and luscious in color.

Curious, I asked why the design team chose to go with a glossy finish for the new Red Zune 4, and Zune 8:

As a design team, we need to be able to make decisions like going with glossy vs matte for red, doing what’s right for the appeal of the color, vs locking  ourselves into an inflexible and rigid design philosophy.

Should we assume the metal plate on the back will make Zunes as scratch-friendly as iPods? And the champion will be the Red Zune?