Look at the design! The deep orange background is incredibly vivid, the yellow is quite pastel-subtle and the black and white is very modish. Extra kudos to the graphic artist behind this one!
This stamp is also part of a series called the “republic” issue. The Union of South Africa had existed since 1910 but it was re-established in 1961 by the Republic of South Africa. A new constitution also came into force. The country decided by 52% to 47% to step away from being ruled by the British monarchy and stand on its own; the role of the Queen was replaced by a State President.
The new constitution also established English and Afrikaans as the state languages (previously English and Dutch had been the official languages but Dutch was ousted). The new stamps reflected this dual language status (as shown above). After 1966, however, the initials RSA were used, as they work in both languages and take up less space.