They're slowly changing from white to black, so I figured that since it's a slow process, why not show them as whatever's "in the middle"? Think of it as "location ethnicity" - North-Western Europe to Southern Europe (the Mediterranean area), then to Africa. Eventually, Laura and whoever's in close proximity to her will eventually turn African, but it's a slow journey to get there.
Frankly, I don't think there's ever been a slow change story for "changing races" ... usually it's instantaneous. I just guessed that this would be an interesting way to go about it.