I know this one is gonna be a difficult one, but nevertheless, maybe somebody has come across a sweet tweak to change the Windows 8 Login Screen Backgroung (or color) If so please share it the community!
DISREGARD: Argh, don't know how I misunderstood your request. I've got to go to bed. What worked for me was what I did when I installed Windows 8 Consumer Preview. Google for "tweakslogon.zip", last version I know of was v1.2. With this program I was able to change that background. It was intended for Windows 7 but it worked for me with the CP. Then I used a Microsoft Account to logon and to sync. Now every computer I sync with that account automatically gets that background. If there's another easier way someone please share. I still have the RP and can probably get the CP too if this trick doesn't work under the RP, but it's a little pain to do for everyone's PCs. If I remember correctly, the RTM blocks this program, but the CP definitely didn't. Edit: I do recall attempting to follow someone's directions to change the picture manually, and it didn't work for me. I wasn't able to delete the old picture or rename it, so I couldn't replace it with my own, and this was after taking ownership of the file/folder.