Add a dynastic rule “Location” with text as follows:

Each Sheep has exactly one Location, tracked in the GNDT . The eligible locations are CH, UK, OZ and NZ. Each Sheep begins in Center Field.  As a weekly action, a Sheep may change his own Location to any other Location, subject to any restrictions on movement or on the availability of any location.
If there are at least three White and no Black sheep in a given Location, any of the White sheep in that location may declare (via blog post)that location off-limits to any Black sheep, whereupon no Black sheep may enter that Location until the restriction is lifted. If there are at least three Black and no White sheep in a given Location, any of the Black sheep in that location may declare (via blog post) that location off-limits to any White sheep, whereupon no White sheep may enter that Location until the restriction is lifted. The restrictions in the preceding two sentences are automatically lifted at 00:00:01 on Sunday. A Sheep of undetermined color who, while in a particular location becomes Black or White does not count as being a Black or White sheep “entering” that location by reason of attaining that color.