If the latter what for? How did taking colonies not go against communist doctrine
Russian interest in Africa, especially the Horn of Africa where Italy was most active, actually predates the Soviet era. During the second half of the nineteenth century Britain expanded its influence in eastern Africa in a way that soon became an obvious could be a threat to Tsarist Russia's trade interests, especially in terms of the Suez Canal and the access it provided to the Red Sea. Most notably, during the Russo-Turkish war of 1877, a war that Britain opposed, the Suez Canal was closed off to the Russians, which emphasized how British control of Egypt and the Canal threatened Russian interests.
Tsarist Russia's interest in Africa initially focused on Ethiopia. The Russians believed that control of Ethiopia, and thus the Blue Nile tributary of the Nile River, would potentially allow them to cut off the flow of the Nile towards Egypt entirely and thus render British colonial holdings there essentially worthless. Russia also looked at Ethiopia in more traditionally colonialist terms however. While other European colonial powers generally disregarded the areas Russia believed that it had both a bounty of goods (coffee, cotton, butter, honey, hides and meat) to offer and a geographic composition that would make it easily exploitable. Rather than colonization by force however, Russia in stead entered into an informal alliance with Ethiopia, driven at least in part by the fact that both the Russian and Ethiopian royal families were Orthodox Christians. By the end of the nineteenth century Russia was providing arms and military support to Ethiopia's emperor Menelik II. When Italy invaded Ethiopia for the first time in 1895 the Russians provided not just military but humanitarian aid, and were instrumental in a crushing and humiliating defeat for the Italians. Finally, Ethiopia was also a favourite subject of many Russian academics during this time. As a result, many aristocrats and academics from the defeated White Army ended up in Ethiopia following the post-revolution civil war.
Immediately following the 1917 revolutions and the establishment of the Soviet Union there was initially very little Soviet interest in involving themselves in African affairs. With an internal civil war still raging, as well as perceived threats in capitalist Europe, the Russo-Ethiopian relationship was considered of very little importance. During the early years of the Soviet Union they mainly practised what they called 'sabotaging imperialism from the rear.' This essentially involved undermining the colonial regimes of capitalist nations in Africa by supporting local nationalist leaders and movements. During this time the Soviet Union attempted to re-establish ties with Ethiopia, although they were blocked from doing so by French and British interference. Formal Soviet involvement in Africa at this time was limited to relatively small scale trade. Interestingly enough, during the second Italian-Ethiopian war Soviet Support was limited mostly to speeches in the League of Nations General Assembly, with none of the practical support Tsarist Russia had given four decades earlier.
The rise of fascism in Europe and the alliance between Hitler's Germany and Mussolini's Italy forced the Soviets to re-evaluate their diplomatic priorities. By the mid-thirties France and Britain looked to be Russia's main allies against an eventual fascist invasion. As such the fascists opposed to abandon their involvement with African nationalists, as subverting the colonial regimes in Africa would mean the economic and military weakening of potential allies.
It was the German invasion of the Soviet Union that marked a change in the perceptions of the Soviet Union in Africa. With Germany coming within twelve miles of Moscow Stalin's regime took a new tack in their propaganda, calling back to the great Tsars of Russia's history and even restoring the Russian Orthodox Church. This proved to be the impetus for emperor Haile Selassie to re-open formal diplomatic ties with Russia in 1943.
By this time most territory bordering Ethiopia was controlled by Britain. This led to the remarks made by Stalin at the Yalta Conference that you based your question on. Stalin hoped that with the Soviet Union taking an active role in Ethiopia it could expand its influence over Africa. Ethiopia was not unopposed to that idea. British control of territories bordering Ethiopia, most notably Eritrea and Somalia, came with an increase of political freedoms for its native populations when compared to the prior Italian occupation. This resulted in a flurry of political activity that threatened to spread to Ethiopia in a way that would destabilize Haile Selassie's rule.
In the end however, it was the United States that turned out to be the most important political ally for Ethiopia. By 1944 the two countries established formal diplomatic ties. The United States had several reasons for its involvement with Ethiopia. It represented a potential air travel hub and major communications centre in the area. Furthermore, with Ethiopia as the oldest independent nation on the African continent, it gave the United States the ability to gain a strategic and diplomatic foothold in a continent that in the past decades had been the almost exclusive domain of European colonial powers. In turn, Ethiopia hoped that closer ties to the United States could be used to dissuade Britain from keeping control of the Haud and Ogaden regions that Ethiopia had hoped to gain control of. Furthermore, American investments in the modernization of Ethiopia that Haile Selassie pursued came with far fewer ideological strings attached than possible Soviet investments.
Finally, ideological shifts within the Communist Party following World War II led the Soviets to abandon their involvement in Ethiopia. The predominant world view in Kremlin foreign policy at the time hinged on a stark distinction between socialist and capitalist regimes. Unless a developing nation such as Ethiopia explicitly pledged themselves as socialist they were by definition capitalist and thus automatically reactionary allies of the imperialists. Following World War II ties between Ethiopia and the United States were so strong that Haile Selassie actually sent a battalion of his personal body guard to fight on the side of the United States in Korea.
So when Stalin made his remarks regarding possible Soviet control of former Italian colonies he was mostly concerned with using Ethiopia as a launching pad for a post-war decolonization effort in Africa, most likely through a revival of the early to mid 1930s Soviet strategy of supporting African nationalism to destabilize French and British control of the area. However, a combination of a shift in Soviet world view and Haile Selassie allying Ethiopia with the United States rather than allowing his country to be indirectly controlled by either Britain or the Soviet Union meant that Soviet interest in the former Italian colonies was essentially over.
This doesn't mean that the Soviet Union was entirely uninvolved in Africa during the fifties and sixties. With decolonization taking place throughout Africa without any direct Soviet support many of these new African regimes were openly courted by either the United States or the Soviet Union. Either side would often provide informal (or even clandestine) support to various regimes or movements seeking to overthrow those regimes as political necessity dictated.
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