How can I change the purple background color of the lock screen on Ubuntu 17.10?

From these steps here, you can change both the image and the background colour:

1. Move your favorite image to /usr/share/backgrounds:

   • It's better to move the image you want to set as login background to system pictures folder: /usr/share/backgrounds. To do so, open terminal via Ctrl+Alt+T, then either run command:

     sudo cp /PATH/TO/IMAGE /usr/share/backgrounds/
     

2. Edit the css file that define GDM login background:

   • Open terminal (Ctrl+Alt+T) and run command to edit file /etc/alternatives/gdm3.css, which is linked to /usr/share/gnome-shell/theme/gdm3.css:

     # Make backup first before editing
     sudo cp /etc/alternatives/gdm3.css /etc/alternatives/gdm3.css.bakup
     sudo nano /etc/alternatives/gdm3.css
     

     

3. When the file opens, go to menu (right after ‘Save’ button) -> find and find out following section:

   #lockDialogGroup {
     background: #2c001e url(resource:///org/gnome/shell/theme/noise-texture.png);
     background-repeat: repeat; }
   

   change the lines into:

   #lockDialogGroup {
      background: #000000 url(file:///usr/share/backgrounds/Aardvark_Wallpaper_Grey_4096x2304.png);
      background-repeat: no-repeat;
      background-size: cover;
      background-position: center; }
   

   Replace Aardvark_Wallpaper_Grey_4096x2304.png with your picture file name, and or change the hex colour code ##2c001e to the color of your choice.Here I used black.

   

Lockscreen background:

Note: Backup all files before changing them

Run: sudo update-alternatives --config gdm3.css and choose the option 1 (or the one with the path to gnome-shell.css file) as seen below:

Reboot and Ok, you're good to go.