Changing the Windows 10 log-in screen to a solid background color requires that you edit the Windows Registry. Windows includes a Registry Editor for this purpose. The instructions for editing the registry should be followed precisely. If you are uncomfortable with making such modifications, you may wish to pass on making this change to your computer.

   
  • Open the Registry Editor. To do so, type run into the windows search bar to the right of the Start  button.
         
  • The Run application will launch. If you encounter User Account Control (UAC), input an admin password and click or tap Yes to continue. Type regedit into the Open input field, and then click or tap OK.
         
  • The Registry Editor will launch. In the Registry Editor, expand the folders and carefully locate this path: HKEY_LOCAL_MACHINE -> Software -> Policies -> Microsoft -> Windows -> System. Right-click on System. In the right-click menu, click New. In the sub-menu, click DWORD (32-bit) Value.
         
  • Name the new value: DisableLogonBackgroundImage
         
  • Right-click on DisableLogonBackgroundImage, and click Modify. In the Value data field, change the 0 to a 1, then click OK.
         
  • By changing the value to 1, you have instructed Windows to display a solid background color for your log-in screen from now on. If you wish to revert back to the default Windows 10 log-in screen, set the value back to 0. Next time you log into Windows, the background color of the log-in screen will be a solid color. The color is based upon your Windows accent color configured under Settings -> Personalization -> Colors, which could also be automatically selected from your background.
         

Source: https://techhelpkb.com/how-to-change-the-windows-10-log-in-screen-background/