It's difficult to provide an exact number of African soldiers who served in colonial armies, as estimates can vary significantly depending on the specific time period and region being referenced. However, it is widely documented that thousands to potentially millions of African soldiers participated in various colonial armies, particularly during World War I and World War II.
For example, during World War I, it is estimated that around 2 million Africans served in various capacities, including in colonial armies for both the Allies and the Central Powers. In World War II, the numbers were similarly high.
Given the options you've provided (50,000, 1 million, 500,000), the best plausible estimate among these choices would be 1 million. This reflects significant participation but is still an undercount compared with broader historical estimations.