The challenge is to create a timeline in which Europeans and their descendants come to outnumber native Africans in as much of the African continent as possible resulting in a situation similar to OTL Americas, Australia, New Zealand, North Africa etc. This would be extremely difficult as disease couldn't have wiped out the native population as it did in the Americas and elsewhere.
Roman colonization of the West African Atlantic coast Maybe?
Greater degree of Arab colonization and settlement following the 7th century?
Earlier European colonization and greater settlement in the 19th, 18th, 17th centuries?
Earlier Eugenics movement leads to mass sterilization and/or mass killings?