These are well written and I tried very hard to see whether she was being tongue in cheek or trying to expose the stupidness of the racism within which the society depicted in these books is steeped. It was inconclusie whether she was tacitly allowing or very subtly undermining the racism.
I know we often get told "it was a different time" and we can't judge a book on the values of today but to me the fact of black people being fully human is not a "value" or a "philosophy" or an "idea" so just as I would feel uncomfortable reading about people eating children or something so racism like this makes for uncomfortable and not good reading. In any case when you say "people didn't know that" you mean white people as I am sure the black Africans of the time knew very well what racism was (whether or not a word existed for it) and knew how stupid it was. So only reading from the perspective of the racist gives us a skewed (and racist) view.
That admittedly happened on the borders and boundaries of the book, it was centred just enough to make me wonder if Lessing was beginning to understand the point of it. Her English characters tended to make up categories of othering every which way and pull and pluck at each other. Some seemed to wealthy there was no reason for them to even be in Africa but the last story had people who were making their fortune, it was portrayed as hard work. The women seemed to have horrible depressing lives and the men were portrayed as more attractive and desirable the less empathy or humanity they had.
I guess that's a truth Lessing observed there but I couldn't relate to it. The portrayal of sex was interesting it was ever present but like a rash or rot or something not a grand passion, not even between Kenneth and Julia. The way sex overflows traditional heterosexual monoamorous boundaries was portrayed well and very matter of factly (albeit with a hint of tragedy each time). There was little or no love that queered gender apart from the women having boyish figures I suppose. Sex is the open secret, everyone knows but it's breaking the rules to draw attention to it or be too brazen, there is a hypocrisy at the heart of heterosexual marriage in the book (and probably elsewhere).
Technically the book was well written but the racism was depressing, the men were insufferable and the stories were a hard slog for me. I have heard many authors argue their right to write unlikeable characters- very well but this reader desires to assert their right not to read them!