Based on the theory the migrant population of an area will eventually adapt to the new living environment over the course of thousands of years and their physical characteristics including their looks will become identical to the indigenous population of the habitat.
That can explain why the average American despite of having 100% European DNA still looks less white than an average Northwestern European.
The transitioning process is of course slow and happens over the course of several generations to eventually return to the physical characteristics identical to the indigenous habitants.