What is the author's purpose in The Color of Water?

Ostensibly, if you believe the subtitle, the purpose of
the book is to pay tribute to his mother, Ruth McBride.  The story is centered around
the way that his mother worked tirelessly to raise a huge family in very difficult
circumstances.  The story also uses episodes where Ruth recounts her own life and her
own family, making the story even more poignant and
powerful.

I would suggest that his purpose is also to
simply tell what is a very compelling story as most memoirs are set up to do.  McBride
emphasizes the importance and use of education in all of his family members' lives, so
one could argue that it is another purpose.