I assume by "topic" you mean theme--in other words, what is the primary message or focus of this book. The Color of Water by James McBride is one man's journey to discover how, exactly, he fits into this world. He is in search of his cultural identity. This is something most people kind of already know or can generally figure out; however, James has many elements of his life which are confusing to him until he is able to uncover his mother's life story.
James grows up in the projects of New York, and his identity as a black person is fairly well defined by the men in his life. His father was a black man and so was his stepfather. There is no ambiguity to them. When it comes to his mother, though, there are so many ambiguities and she flatly refusesto talk about any of them with her children. It is James's nature that he needs to know her story in order to make sense of his own.
First, she is not black but presents herself as such in the way she talks, the way she behaves, and where she lives. Despite being white, she clearly identifies herself with black women in ways that can be seen. On the other hand, Ruth acts against the black stereotype by insisting her children go to the best schools she can manage for them--almost always white, and often Jewish. This does not fit with who Ruth appears to be.
Second, Ruth is Jewish but raises her children as protestants; her Jewishness is one of the ambiguities about her that James finds unsettling. Third, she values family above all things and is adamant that her children never speak about family matters outside of the family; yet, ironically, she never speaks at all about her own family or about her time before marrying James's dad.
All of these ambiguities are a consistent undercurrent in James's life, and he is one of Ruth's only children to "act out" because of it. As he finally gets his mother to speak to him, James is able to put the pieces together and make some sense of his instincts and intuitions. Once he knows from where his mother came, her rather odd behaviors make more sense to him and in turn help him make sense of his own life.
All of these things are disconcerting to James as he tries to understand his heritage; once his mother tells him the truths of her life, James is able at last to reconcile his feelings with his experiences.