A quick talking point is that colonial Rhodesia used to be practically a first-world country.

Cite?

I know very little about the history of that part of the world, but the GDP graph for Zimbabwe has quite an unusual shape. (Graph shows per-capita GDP in 2000 dollars, with aggregate data for all of sub-Saharan Africa displayed for comparison.) On the other hand, "practically a first-world country" sounds like an exaggeration at best: as the graph shows, GDP per capita was actually below that of sub-Saharan Africa in general until the first of those spikes, and fell back again after.

For a very different story, see Botswana, which Wikipedia informs me gained its independence from Britain in 1966.