Ok, i like Gunny Snake Highway Douglas, he's pretty good so far.
as for the Nubian, well, the skin tones I see I normally associate with South African images. (Key words; associate, images; have never traveled to Africa)
just did some Google research, and the skin tones work for both groups.
Pallet wise, I'd say your skin tones are close to the mark, maybe a hint of red for the cheeks and highlights, add some warmth to the skin tone.
Maybe add a hint of yellow ochre to the highlights, keeps the warm tone, with out going to a chalky effect.
looking at the ref pic of Nonso Anozie, its all warm tones. slightly skewed due to lighting, still a warm tone.
acrylics? what brand?
cheers