What is the general topic (theme) of The Color of Water?

I assume
by "topic" you mean theme--in other words, what is the primary message or focus of
this book. byis 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.