When the narrator said "Grandfather believed that a well-rooted tree was the color of money", he meant that the tree gave Grandfather food for him and his family to survive.
The phrase "Grandfather believed that a well-rooted tree was the color of money" is added to the story because, for the grandfather, trees are able to support people just as money is capable. This is because a tree can provide food, and products to be sold, which will solve all people's needs.
Because of this ability to support a family, the grandfather concludes that the root of the tree must be the same color as money to show its similarities.
