why is he brown?

Here's one of those questions you get asked by an almost five year-old. We were watching YouTube clips of Mamma Mia, and there was the duet between the older, lighter Christine Baranksi and the younger, darker, Philip Michael.

Marta asks: Why is he so brown?

Justin responds: Because he has brown skin.

Marta continues: But he is really brown. Like brown all over.

Justin thinks about how to explain race to a child. Should he take it down the biological route ("you see, we all have this thing called melanin in our skin"), or should he take it down the multicultural route ("all people come with different shapes and colors and sexual orientations and blah blah blah"). But, somewhere inside of me, I could hear Peter Tosh's words ringing out:

No matter where you come from, as long as you are a black man, you are an African/ No mind your nationality, you have got the identity of an African.

So I explained to Marta that the brown man's family came, at some point, from Africa, where the people are mostly brown. Yes, yes, I know that people of European descent, like, say, FW De Klerk, are also Africans, and that brown people, like, say, Barack Obama, are Americans, and so on. But Obama's dad was from Kenya, and De Klerk was of Dutch extraction. So there.

Anyway, Marta seemed to accept this as a plausible explanation. What would you have said?