Why do momma and dad disagree about Birmingham?

Mother insists that Birmingham is a good place, not only because it is warm. Dad disagrees, and refers to the "Coloreds Only" bathrooms and other instances of segregation. The discussion ends, and the Watsons start up their old car (which they call the Brown Bomber) so that they can travel to their Aunt Cydney's house and spend the night in a warmer place.