Here is the data pack that allows you to edit the base diffuse textures for all bundled avatars (and some of the props too). Now you can beautify (or deface) them at your leisure :).
Please read the pdf here https://drive.google.com/folderview?id=0B8KlsC2kpc7nRHJZNWJHY0lyWDQ&usp=sharing about how to use them (It should be pretty simple: just edit them and place them in a certain folder – written in the pdf – and then, when you start FaceRig, the edited ones will be used instead of the stock ones).
What is also cool is that you don’t even have to exit FaceRig to update your progress while you are editing them. Just save the tga from your graphic editing program directly in that custom folder, alt-tab to FaceRig, reload that avatar and the latest brush-strokes will be instantly visible :).
Happy painting 🙂
PS: Yes you can edit the color masks for extra custom patterns the same way too.