ref: http://www.howtogeek.com/223875/how-to-change-the-login-screen-background-on-windows-10/
color only login
open the Registry Editor
“HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\System”
create a DWORD value named “DisableLogonBackgroundImage” with the value “00000001”.
note: Delete the value or set it to “00000000” to go back to the default Windows 10 “hero image.”
set color to login
Settings app, select Personalization,
select Colors,
disable the “Automatically pick an accent color from my background” option.
The color you select here will be used on your login screen if you’ve used the above registry hack.
result
