Even if we broaden our criteria to include all former British colonies, the only one that became a Communist regime was South Yemen (excluding Maurice Bishop's very brief People's Revolutionary Government in Grenada). Compare that to France's and Portugal's colonies, a sizeable number of which did become Communist. Is it just a coincidence that this was the case, or did the British set up institutions in their colonies that were absent in the colonies of other countries which hindered the possibility of Communist takeovers?
Edit: Also just want to clarify, I wish to exclude socialist-leaning regimes like Tanzania which were not nominally Communist.
A case could be made for including Idi Amin's Uganda as a communist state. Of course, it was by no means really communist, but Amin and his government sharply and rapidly increased state control over the economy, and during this regime Uganda was aligned with the USSR and East Germany. This was certainly enough for many other countries to be counted as communist.
For more information on post-colonial Africa as a whole, M. Meredith's The State Of Africa is an excellent general overview.