While looking for alternative back covers for the Diamond I came up with this
http://www.buywirelessnow.com/utstarcom/buy/accessories/cat/327/model/1035_379510/accessory/393538
It's the official UTStarcom accessories site that usually carries OEM items. They list the BTE6950AL extended battery/cover in dark grey.
No pictures but you can assume Alltel will be getting one and it's gonna have a Dark Grey back cover :P
-CC