I was taught that humans evolved on the plains of Africa a long time ago. However every depiction of these early humanoids I come across shows someone with a white-ish skin colour and a lot of hair. Is that historically correct or just plain biased?
This is a noted issue, particularly with older museum exhibits and older texts.
Long story short, except for those species that migrated out of Africa (e.g. Neandertals, Homo erectus, etc.), members of our genus would have had high levels of melanin. I stress this is true only of our genus, because the early members of our genus were the first to begin losing body hair. The loss of body hair is probably related (at least in part) to a need for our savanna-dwelling ancestors to regulate body temperature more efficiently, largely through sweating.
However, a consequence of this loss of body hair was high melanin levels in the skin in order to protect from the dangers of high UV radiation at the equatorial latitudes in Africa inhabited by early members of our genus. This is supported by looking at the melanin levels of modern populations, where "skin reflectance is strongly correlated with absolute latitude and UV radiation levels" (Jablonski and Chaplin 2000). In other words, skin color correlates strongly with latitude (and therefore UV exposure).
Consequently, only members of our genus that moved out of these equatorial latitudes (like neandertals) would have developed depigmented skin (as an adaptation to lower UV levels in more northern latitudes). Homo erectus, for instance, may have had skin colors from across much of the modern range of skin colors. That said, there has been some recent (yet still inconclusive) work suggesting that depigmented skin didn't evolve in our species (Homo sapiens) in Europe until relatively recently, within the last 8000 years.
All that said, there is still the problematic bias of displays that show a "progression" of skin colors from dark skinned earliest members of our lineage to lighter skinned modern humans. There is a lot more awareness these days but the issue is pervasive: just Googling "Human Evolution" will bring up dozens of variations on the famous "march" of human evolution. Almost all examples are either entirely light skinned or they depict Homo sapiens and more recent humans as progressively more light-skinned. For neandertals it makes a certain amount of sense, but for every other member of the genus, a white "default" is hard to defend.
Sources: Jablonski, Nina G., and George Chaplin. 2000. The Evolution of Human Skin Coloration. Journal of Human Evolution, 39(1):57-106.
Juzeniene, Asta, Richard Setlow, Alina Porojnicu, Arnfinn Hykkerud Steindal, Johan Moan. 2009. Development of Different Human Skin Colors: A Review Highlighting Photobiological and Photobiophysical Aspects. Journal of Photochemistry and Photobiology B: Biology, 96(2):93-100.
You may also get a good answer from r/askanthropology