sentences:  When Zanzibar gained independence from the United Kingdom on 10 December 1963, a green disk with two yellow cloves was added to the flag.On 12 January 1964, John Okello overthrew the Sultan of Zanzibar and adopted a black-yellow-blue tricolour as the flag of the People's Republic of Zanzibar and Pemba.On 29 January, the country's flag was changed to a blue-black-green tricolour.