I am using Ubuntu 19.10 and I really like Ubuntu very much, but I really don't like that purple colour everywhere. It was easy to chage desktop theme, but a bit more difficult to change colour of Plymouth boot splash. You can install other available boot splash by installing another packages, or you can create new theme by updating the current one. Here is some help:
This is based on the khAttAm blog which has a bit outdated paths, but otherwise is still working. https://www.khattam.info/howto-change-ubuntu-pinkpurple-plymouth-boot-screen-to-any-color-you-like-2010-11-09.html
sudo cp -R /usr/share/plymouth/themes/ubuntu-logo /usr/share/plymouth/themes/ubuntu-logo-nonpink
sudo nano /usr/share/plymouth/themes/ubuntu-logo-nonpink/ubuntu-logo.plymouth
I have changed these lines:
Name=Ubuntu Logo NonPink
ImageDir=/usr/share/plymouth/themes/ubuntu-logo-nonpink
ScriptFile=/usr/share/plymouth/themes/ubuntu-logo-nonpink/ubuntu-logo.script
sudo nano /usr/share/plymouth/themes/ubuntu-logo-nonpink/ubuntu-logo.script
I have changed these lines:
Window.SetBackgroundTopColor (0.85, 0.85, 0.85);
Window.SetBackgroundBottomColor (0.75, 0.75, 0.75);
Set those numbers as you like - take R, G and B decimal values from RGB code of colour you like and divide them with 256.
sudo gimp /usr/share/plymouth/themes/ubuntu-logo-nonpink/ubuntu-logo.png
Original white logo on light grey backgroud wouldn't look well, so you can invert colours in this picture to get black variant. Or download new picture, for example Tux pinguin, replace that file with the new one (of course you have to rename the new file to ubuntu_logo.png).
sudo gimp /usr/share/plymouth/themes/ubuntu-logo-nonpink/progress-dot-on.png
sudo gimp /usr/share/plymouth/themes/ubuntu-logo-nonpink/progress-dot-off.png
Again, change those as you like. I do not reccomend just to convert those to greyscale, because while the colour is nice, it looks like the dot is shining out grey.
sudo update-alternatives --install /usr/share/plymouth/themes/default.plymouth default.plymouth /usr/share/plymouth/themes/ubuntu-logo-nonpink/ubuntu-logo.plymouth 100
sudo update-alternatives --config default.plymouth
sudo update-initramfs -u
sudo nano /usr/share/plymouth/themes/ubuntu-logo-nonpink/ubuntu-logo.grub
sudo update-grub
search for #lockDialogGroup
sudo nano /usr/share/gnome-shell/theme/gnome-shell.css
search for #lockDialogGroup
sudo nano /etc/alternatives/gdm3.css
background: url(file:///usr/share/backgrounds/mybackground.png);
background-repeat: no-repeat;
background-size: cover;
background-position: center;