This page: https://kb.wisc.edu/page.php?id=27428 appears to answer your question.
If that's not it, try this - I've used this method personally.
http://www.eightforums.com/tutorials/15554-sign-screen-change-background-color-windows-8-a.html
Method one changes pictures, but I'm not as sure it will work:
Create the key HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Personalization if it does not exist.
Make a new string LockScreenImage
.
Set it's value equal to the path to your image file. For safety, I'd recommend a path without spaces.
Method two changes colors but not pictures:
From the Start Screen, type run and press Enter.
Type regedit in the open text box and click OK. If prompted by User Account Control, click Yes.
In the Registry Editor, navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Accent.
Within Accent, check to see if you have a DWORD value titled DefaultColorSet.
If this value already exists, proceed to the next step.
If this value does not exist, right-click somewhere in the empty space within the right-hand pane for Accent, and click New > DWORD (32-bit) Value. Type DefaultColorSet and press Enter.
In the right-hand pane for Accent, double-click on DefaultColorSet to edit the value.
In the window that appears, select the Decimal radio button and enter a new number in the Value data: field. See the chart below for color values: