Comment 2 for bug 1175675

You must remove the PPA version (1.1.0) from your computer fully before downgrading to the 12.04 version (1.0.0). Open a terminal, and type:

  $ sudo dpkg -P mypaint-data-hires mypaint-data mypaint-data
  $ sudo apt-get install mypaint

You can also do this though Synaptic (search for all "mypaint" packages, and purge or remove them). I can't remember whether the Ubuntu Software Centre allows you to remove dependencies like this easily.

Alternatively, you can continue using the PPA version by following the instructions at https://launchpad.net/~achadwick/+archive/mypaint-testing