Afaik, skin tone, hair texture, and facial features make a large difference in how African Americans treat each other.

White people, in my experience, are apt to think of race in binary terms, but this might imply that skin tone affects how African Americans actually get treated.

I'm still not confident because we're not, as Nancy mentioned, completely binary about race even in the US.

What you'd really need to do is a comparative study between the US and somewhere like Brazil or Cuba, which had a different history regarding mixed race. (The US worked by the one-drop-of-blood rule; Spanish and Portuguese colonies had an elaborate caste system where more white blood meant more legal rights.) If it's mainly a cultural distinction, we ought to see a major difference between the two countries -- the light/dark gap should be larger in the former Spanish colony than it is in the US. If culture doesn't matter much, and the gap is purely genetic, it should be the same all around the world.

The other thing I would add, which is easy to lose track of, is that this is not research that should be done exclusively by whites, and especially not exclusively by whites who have an axe to grind about race. Bias can go in that direction as well, and a subject like this demands extraordinary care in controlling for it. Coming out with a bad, politically motivated IQ study could be extremely harmful.