Windows 8 start menu and the logon screen has the same color background and this option is not changeable in the developer preview build, although you can do it with registry and other tweaks. Windows 8 Start Tweaker is a freeware app for Windows 8 which lets you change the start screen background wallpaper as well as the logon screen background color.
You can select any image file (png/bmp/jpg/gif) as your default screen background, and you can also change the background color of your Start screen or logon screen.
Once selected, you need to click the apply button and the app will log off Windows 8 with a warning. After logon, you can see the new wallpaper and color in action.
Logon screen will have the background color you selected.
If you want the image fills out the whole screen, you should make a image by yourself. The Height of the image should be doubled with the Height of your Screen Resolution. For example, if your Screen Resolution is 1280 X 800, you should make a image with Height 1600, and the width is not limited.