why did ruth fall in love with a black man?

No one chooses who they fall in love with, and at the time Ruth fell in love, having a relationship with a black man was a very dangerous thing to do. This subject is broached in gradesaver's chapter 9-12 summary for the novel;

"Ruth was a rebellious girl who hated her father, and since her father hated blacks - and especially black men - it made sense for her to fall in love with a black man. In those days, she explains, interracial relationships were intensely dangerous, as black men were hanged in the South for even looking at a white woman. Ruth longed for romance, but no one ever asked her on a date until, one day, a young black man began visiting the store and asking her how she was doing. She had always liked the black people in the town because they never judged her."

If you'd like to read the rest of this excerpt simply follow the link provided below;

http://www.gradesaver.com/the-color-of-water/study-guide/section3/