make it possible to configure grub background color in a usable way

Maybe this could be handled with the update-alternatives system like the newt palette?

Possibly. It will take a bit of rearrangement, so pushing out to final.

I've changed plymouth-theme-ubuntu-logo to add /lib/plymouth/themes/ubuntu-logo/ubuntu-logo.grub, which is linked to the default.plymouth alternative like this:

        update-alternatives \
                --install /lib/plymouth/themes/default.plymouth default.plymouth \
                /lib/plymouth/themes/ubuntu-logo/ubuntu-logo.plymouth 100 \
                --slave /lib/plymouth/themes/default.grub default.plymouth.grub \
                /lib/plymouth/themes/ubuntu-logo/ubuntu-logo.grub

I've left derivatives alone, but they can emulate this on their own schedule. Note that Plymouth .script files represent RGB values as decimals between 0 and 1; multiply by 256 and round to get values to pass to the GRUB background_color command.

This bug was fixed in the package plymouth - 0.8.2-2ubuntu22

---------------
plymouth (0.8.2-2ubuntu22) natty; urgency=low

  * Install slave alternative for plymouth-theme-ubuntu-logo, telling GRUB
    to set an aubergine background (LP: #696915).
 -- Colin Watson <email address hidden> Fri, 15 Apr 2011 14:35:17 +0100

This bug was fixed in the package grub2 - 1.99~rc1-13ubuntu1

---------------
grub2 (1.99~rc1-13ubuntu1) natty; urgency=low

  * Resynchronise with Debian. Remaining changes:
    - Adjust for default Ubuntu boot options ("quiet splash").
    - Default to hiding the menu; holding down Shift at boot will show it.
    - Set a monochromatic theme and an aubergine background for Ubuntu.
    - Apply Ubuntu GRUB Legacy changes to legacy update-grub script.
    - Fix backslash-escaping in merge_debconf_into_conf.
    - Remove "GNU/Linux" from default distributor string.
    - Add crashkernel option.
    - Bypass menu unless other OSes are installed or Shift is pressed.
    - Allow Shift to interrupt 'sleep --interruptible'.
    - Reduce visual clutter in normal mode.
    - Remove verbose messages printed before reading configuration.
    - Suppress kernel/initrd progress messages, except in recovery mode.
    - Handle filesystems loop-mounted on file images.
    - Ignore devices loop-mounted from files in Linux grub.d scripts.
    - Show the boot menu if the previous boot failed.
    - Don't generate device.map during grub-install or grub-mkconfig.
    - Adjust upgrade version checks for Ubuntu.
    - Suppress "GRUB loading" message unless Shift is held down.
    - Adjust versions of grub-doc and grub-legacy-doc conflicts.
    - Fix LVM/RAID probing in the absence of /boot/grub/device.map.
    - Look for .mo files in /usr/share/locale-langpack first.
    - Build-depend on qemu-kvm rather than qemu-system for grub-pc tests.
    - Add a grub-rescue-efi-amd64 package.
    - On Wubi, just update wubildr rather than asking for an install device.
    - Check hardware support before using gfxpayload=keep.
    - Put second and subsequent Linux menu entries in a submenu.
    - Preferred resolution detection for VBE.
    - Set vt.handoff=7 for smooth handoff to kernel graphical mode.
    - Add grub-mount-udeb, containing just grub-mount.
  * Use /lib/plymouth/themes/default.grub if it exists, rather than
    hardcoding an aubergine background (LP: #696915).

grub2 (1.99~rc1-13) unstable; urgency=low

  * Cherry-pick from upstream:
    - Use correct limits for mips initrd.
  * Run grub-install on install or upgrade of grub-yeeloong.
  * Update branch_fuse.patch:
    - Tell FUSE to run single-threaded, since GRUB code is not thread-safe
      (LP: #756297).
 -- Colin Watson <email address hidden> Fri, 15 Apr 2011 14:37:43 +0100