The colonial policy of assimilation was introduced in Africa by the French. Assimilation was a colonial policy under which the French sought to absorb the peoples of their colonies into French civilisation by making them adopt French culture, language, laws, customs, and political institutions. The ultimate goal was to transform colonial subjects into "Black Frenchmen" who would be culturally indistinguishable from citizens in metropolitan France.
Under the policy of assimilation, Africans in French colonies (particularly in the Four Communes of Senegal - Saint-Louis, Goree, Rufisque, and Dakar) who met certain criteria could acquire French citizenship. They were expected to speak French, adopt French dress, practice French customs, and be educated in the French system. Those who met these requirements were called assimiles (assimilated persons) and enjoyed the same rights as French citizens.
The other colonial powers pursued different approaches. The British used the policy of indirect rule, governing through existing traditional institutions. The Germans employed a mix of direct and indirect methods. The Portuguese had a similar but distinct policy called the assimilado system, but the question specifically asks about the policy of assimilation, which is most closely and primarily associated with the French colonial approach.