There are six chromosomes with strong coding for melanin in skin in humans. It's not a dominant or recessive trait, the more you have of the gene the darker you are.
A very pale northern European has the genotype ww,ww,ww,ww,ww,ww resulting in entirely pale skin. A very dark skinned African has the genotype BB,BB,BB,BB,BB,BB resulting in extremely dark skin.
A first generation hybrid between these two populations always results in Bw,Bw,Bw,Bw,Bw,Bw, about half as dark as the African parent.
But if two first generation hybrids interbreed you roll the dice, and the result is random, and can be anywhere between the two. Their child might be Bw,BB,BB,Bw,ww,Bw and be slightly darker. Or it could be ww,Bw,ww,BB,ww,Bw and be slightly lighter. Or it's possible a given child could be BB,BB,BB,BB,BB,BB or ww,ww,ww,ww,ww,ww all over again.
It's just chance.