This means that, for the first time, she understood her color difference, because she was treated differently.
Explanation:
According to Zora Hurston's memories, she was 13 years old when she left her hometown to study in another school. As a black girl living in the South, maybe it was common to hear racism. But it was not. She said that her small town has a little white population, and her childhood was "normal". But when she got in touch with other children (the white ones) in another school, she realizes the color differences, and how Jim Crow laws were cruel.