It's likely you're thinking of James McBride himself, the author of the book.
"The Color of Water" is a memoir, and the main characters are:
* James McBride, the author himself, who is telling his story.
* Ruth McBride Jordan, his mother, who is the central figure in the narrative.
* Andrew McBride, Ruth's husband and James' father.
The book is about Ruth's life, her journey from a young Jewish girl in North Carolina to a black woman raising a family in New York, and how she coped with racism, loss, and the complex identity of being both Jewish and black in a segregated America.
If you're looking for a specific character named "Peter," it's possible you're thinking of a different book or film.