Or are they, actually. The human development index [HDI,1] can serve as a proxy for "prosperity", but I'm having a hard time finding the "degree of colonization" each country suffered, to see if the two are anti-correlated. The closest I could find was that Ethiopia and Liberia are considered to never have been colonized [2], but both are near the bottom of the HDI list. Maybe duration or how recently colonization ended could be used instead.
[1] https://en.wikipedia.org/wiki/List_of_African_countries_by_H...
[2] https://www.thoughtco.com/countries-in-africa-considered-nev... (I found it amusing how the article says Turkey was never colonized by the West. Yes. In fact, a large chunk of Europe was under Turkish colonial control [3]. One could argue a large chunk, Byzantium, still is, but is no longer considered Europe due to being conquered.)
[3] https://en.wikipedia.org/wiki/Territorial_evolution_of_the_O...