Removing/hiding all boot screens

Question

DISCLAIMER: I'm new to Ubuntu (18.04 - no dual boot).

I'd like to remove all colored screens after booting and turning off (while not seeing any code run on the screen) - also changing the login screen background to black (not lock screen).
After the manufacturer logo there's a second long purple screen. Then the screen totally turns off and after a second black screen the ubuntu loading screen shows up. Finally I can log in.
I searched for many questions and there are some thing I tried:

• Editing GRUB file - removing quiet splash/replacing to only quiet (I remember I could remove the splash, but then there was the code running on screen, so I'd like to change the color to black) - then updating grub
• Changing plymouth theme file to change the Window.SetBackgroundTop(Bottom)Color to (0,0,0) - this one hasn't changed anything I guess
• Creating a new plymouth theme and using a total black 1920x1080 image as background - haven't noticed any change, even tho the theme was used as alternative

Some of these changes messed with plymouth's Reboot and Power Off services and couldn't solve it, so I had to reinstall Ubuntu. I hasn't tried anything yet.

Is the first purple screen GRUB and the booting plymouth? I'm not sure..

Answer 1

to achieve the requirements in the Question for default Ubuntu 18.04

need to edit the below files:

1. for changing grub color: /usr/share/plymouth/themes/ubuntu-logo/ubuntu-logo.grub
2. for plymouth color: /usr/share/plymouth/themes/ubuntu-logo/ubuntu-logo.script
3. for login-screen background color: /usr/share/gnome-shell/theme/ubuntu.css

ubuntu-logo.grub my file content. replace the values to 0,0,0,0; for black

if background_color 66,48,228,0.5; then
  clear
fi

ubuntu-logo.script some of my file content. replace the values to (0, 0, 0); for black

Window.SetBackgroundTopColor (0.16, 0.00, 0.12);     # Nice colour on top of the screen fading to
Window.SetBackgroundBottomColor (0, 1.02, 1.02);  # an equally nice colour on the bottom

ubuntu.css my file edited content at around line number 1814

#lockDialogGroup { background-color: #000000; }

after editing the files, save & close all files..

then run sudo update-grub

Reboot to see the change.