Without any real criteria on which to base a decision, the casting of James McBride and his mother Ruth for a film verison of The Color of Water really comes down to personal choice.
For James McBride, three actors stand out as good choices: 1. Taye Diggs -- he has serious acting skills, and he could capture the inquisitive nature of McBride well. 2. Omar Epps -- like Taye, Omar has a seriousness that would suit the role well. 3. Rob Brown -- most recently in The Express, Brown has already played a writer in Finding Forrester.
Ruth would be a bit of challenge to cast. However, three actresses would do nicely: 1. Embeth Davidtz -- she has been in numerous films, including Schinlder's List and Junebug. There is an intensity to her that would suit Ruth well. 2. Phoebe Cates -- she has Ruth's sense of playfulness, and that's an important component of her character. 3. Lisa Edelstein -- most famous for her role on House, Edelstein would already have an established chemistry with Epps, which would add an nice emotional aspect to the discussions James and Ruth have at the end of the book/movie.