We all know that Veronica Roth loves to answer fan questions on her tumblr. Recently, she addressed a question about Four’s ethnicity:
Here is how Evelyn, Tobias’s mother, is described: “She has curly black hair and olive skin. Her features are stern, so angular they almost make her unattractive, but not quite. …At that moment I realize that he and the woman have the same nose— hooked, a little too big on her face but the right size on his. They also have the same strong jaw, distinct chin, spare upper lip, stick-out ears. Only her eyes are different— instead of blue, they are so dark they look black.”
Evelyn is a POC. I don’t know if Marcus’s skin tone is ever described, but I have always pictured him as white and it seems likely, given that he has blue eyes. (Which is not necessarily a given, with blue eyes, but it’s more common.)
You can read more of her Q&A in this compilation here (as well as the full answer to this question).