Regarding mixed-race populations, South Africa's coloureds are the most notable due to their large number (4.5 million). As mentioned before, this is due to the earlier colonisation.
Cape Verde is another example, there the majority of the people (2/3) are of mixed European and African heritage. In Angola too there are perhaps 200,000 mixed-race individuals. In that country they form much of the country's political ruling class (MPLA). Sao Tome and Principe also has a significant proportion of its population being of mixed African and European heritage.
There is a theory that racially mixed societies generally occurred as the result of few European women accompanying men to the colonies. This was overwhelmingly the case in colonial Spanish America. In Brazil too, only 1/10th of the settlers before 1700 were Portuguese women. Once the numbers of European women reached that of European men, the mixed race numbers decreased rapidly as people tended to marry their own kind.
The Portuguese colonies in Africa tended to be the most racially mixed with little institutional racism and until the 1940s, Angolan mesticos played an important economic role in the country, especially in cities like Luanda and Benguela where they often served as middlemen between the Europeans and Africans. However, beginning in the 1940s, the Portuguese government sought to bring more European settlers to Africa and the position of the mesticos as the was eroded by 1974. Also, settler men tended to outnumber women 2 to 1 until the 1940s in Angola, so this often meant that Portuguese men had no choice but to marry with an African or Mestico woman. However, this would quickly change as seen below.
Females as a percentage of Europeans entering Angola vs Mozambique
1943-1949 36% vs 40%
1950-1959 41% vs 43%
1960-1969 49% vs 46%
1970-1974 51% vs 50%
In the other settler colonies, such as Southern Rhodesia and Kenya, the ratio of women to men in the European settler population reached parity or near parity much earlier (1920s). Also, the institutional racism found in those colonies discouraged race mixing. This meant that Southern Rhodesia had only around 6,000 coloureds in 1951 and 15,000 by 1969.
Ratio of Europeans to Mixed Race
Southwest Africa (1970) 3 to 1
Angola (1974) 4 to 1
Mozambique (1974) 5 to 1
South Africa (1970) 6 to 1
Southern Rhodesia (1969) 15 to 1
Kenya (1962) 16 to 1
Belgian Congo (1958) 22 to 1
Northern Rhodesia (1961) 36 to 1
One interesting thing to note above is the position of Namibia, it had a large mixed race population including the Rehoboth Basters, coloureds who had migrated northward from Cape Colony beginning in the 1870s. Finally, if we look at the Belgian Congo and Northern Rhodesia these two territories were different from the traditional settler colonies. Most Europeans here either worked for large companies (mining or agricultural comopanies) with ties or were missionaries or administrators. They often came to Africa with their families and there was little race mixing because of this. In the case of the Congo, many resembled expats more than settlers as they'd often return to Belgium after a few years.