Question

what did the narrator do which change the color of Nicola's face

Answer 1

Seeing them always working so hard at various jobs, the narrator one day dared to ask them why they spent so miserly on their personal needs such, as food, clothing, etc, in spite of earning quite some money. He inquired of them what they did with their money. These questions changed the colour of Nicola's face.   

Answer 2

When narrator asked to Nicola andJacopo you were do this from money then there colour of face changed suddenly.