Recently Microsoft has released the developer preview of Windows 8, its free and anyone can download it and test it his system, as it’s a pre release so it may come with some bugs, If you had installed it on your system and wanna change the background color of Windows8’s Logon Screen and Start Screen.
Ruanmei at DA has created Windows 8 Start Tweaker which allows you to change the background image or color of Windows 8 start screen or logon screen very easily.
Using Windows 8 Start Tweaker you can apply any image with format of PNG, JPEG, GIF, Or BMP.
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*800, you should make a image with Height 1600, and the width is not limited.