Lupita is lighter than your average South Sudanese but she's not so exceptionally light that people would make note of it -- as she's well within the range of variation; there are Nilotics with a red tinge and virtually every family has them.
My half sister has the same skin tone as Lupita, if not lighter.
Don Cheadle's skin tone wouldn't stand out and neither would the average West African; southern Africans, Horn Africans, Igbo and Fulanis would stand out, but the rest of the continent wouldn't really turn heads.
The Dinka and Nuer constitute almost 60% of South Sudan's population but there are other ethnicities that do look different.
We have an entire State (Western Equatoria) where the people are lighter than the average West African. WES is home to the Zande -- a Bantu Nationanality that is the 3rd largest after the Dinka and Nuer.
Zande
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We also have a dozen or so ethnicities in Western Bahr El Ghazal that are much lighter than the Nilotic Dinka-Nuer; these people are collectively called 'Fertit', and they are a Central Sudanic population.
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