"As you know, I think what you're referring to is the fact that professor Warren claimed she was a Native American, a person of color -- And as you can see, she's not."
Now that sounds like a great gotcha line. Unless, you know really anything about the modern Cherokee Nation. In colloquial terms at least, most Cherokees, certainly many, "look white."
So for example, here's Principal Chief Bill John Baker of the Cherokee Nation.
Here's perhaps the most famous Cherokee of the 20th century: Will Rogers.
Now, there's a whole complicated debate about what it means to "look white" and obviously there are different hues of Cherokee in the US today. But I think the examples above speak for themselves. Brown apparently thinks that if Warren were a real Cherokee she'd be over there at the podium with dark-hued skin, war paint and a feather headdress.
Whether Warren was right to claim some level of Indian ancestry or whether she's credentialed with this or that tribe I don't really know or care about. But this "she don't look Indian" line Brown is now basing his campaign on is ignorant to the point of offensive.