What is McBride's main argument in The Color of Water? I do not understand what it is. At first I thought about racism, but then it goes on about...

I think
that McBride's main argument is closely tied to his title, and that while most would associate
that title with race, McBride intends for his readers to realize--as he did--that in regards to
race, religion, and education, humans cannot think everything is either black or white.  The
mature McBride looks back on his life and realizes that if everybody viewed every area of the
human existence clearly (the color of water) and neutrally, then we would have far fewer
controversies, whether they be connected to race, religion, etc.

In regards
to the second part of your question, McBride has to discuss religion and education because they
played just as significant a role in his life as his mixed ethnicity did.  While his mother was
white, and that certainly created controversy for her and her children, she was also Jewish, and
her family would not have approved of her marrying anyone (black or white) who was not Jewish.
Similarly, when McBride's mother seeks to get the best education possible for her children, she
must combat not only segregation based on race but also differences in religious philosophy with
the schools.