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I *think* I know how they did this
https://www.youtube.com/watch?v=CUzSgPkiKcQ&feature=related
They are not actually painting anything onto the walls, not even white or green paint.
I think what i’m seeing here is, they put some kind of a wireless mouse on the roller, which is uncovering layers in a paint program. The stop-mo serves two purposes: Long exposure time gets a brighter, more saturated image out of even a relatively weak projector, and it also covers small positioning errors in the animation and mouse-uncoering action.
You could theoretically skip the projector as well, if you use xyz positioning date from the roller to drive an AE 3-d comp in post.
Did I guess right?