At this time this is not possible due to bug #1291359.
This issue is still present up to Ubuntu 16.04.
The most common thing to do at this moment is to show the default login screen/color.
You can do this by installing dconf Editor.
Open a terminal with Ctrl+Alt+T and type:
sudo apt install dconf-editor
or click dconf Editor to auto-open Software Center and click Install to install it.
Now we must grant LightDM permissions to access dconf Editor.
- Gain root access, type your password (no visual feedback),and Enter.
- Allow user to create a connection to the X server for lightdm
- Switch to user lightdm
su lightdm -s /bin/bash
- Start dconf Editor
On dconf Editor, navigate to
com > canonical > unity-greeter in the left pane.
- Disable (untick) the value for 'draw-user-backgrounds' on the right pane.
- While at it, you may want to disable 'draw-grid' (it disables the dotted grid layer).
Reboot to confirm changes.
Note: from now on LightDM has permission to access the X server, so every time you open dconf Editor, LightDM already has access.
To revoke this access, issue the command
xhost -SI:localuser:lightdm as root.