Well, according to Microsoft you cannot currently change the background image on Windows 8.1 Login Screen. I confirmed this with Microsoft Partner Support HERE . I believe this will change in future versions of Windows.
That being said you can force a specific colour (or color for our American friends!) by using Group Policy:
- COMPUTER CONFIGURATION > POLICIES > ADMIN TEMPLATES > CONTROL PANEL > PERSONALIZATION
- FORCE A SPECIFIC BACKGROUND AND ACCENT COLOR
Alternately you could set the following registry entry:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Accent
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