Straight up stole this from http://winaero.com/blog/disable-logon-screen-background-image-in-windows-10-without-using-third-party-tools/. Just took it and made it into a .reg file.
Download and run disable-windows-login-bg.reg.
This will change the regestry key DisableLogonBackgroundImage under HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\System from
"DisableLogonBackgroundImage"=dword:00000000
to
"DisableLogonBackgroundImage"=dword:00000001
You should end up with this, a solid background depending on your accent color:
