How can I disable the purple "Ubuntu" splash screen that displays at boot before the login screen has loaded? I'd prefer to just see plain text scroll by.
Temporarily disable splash
You can edit the boot entries in the menu from within GRUB to temporarily disable the splash screen.
Try the following: Select the entry, press the key to edit this entry (it should be e), go to the kernel/linux line and remove
Here are two screenshots from a VM to show you how it should look like:
Permanently disable splash
The permanent equivalent was already quoted in Riccardo Murri's answer: