What do you understand by the following statements?

He coloured deeply under his sunburn, then grew pale.

Once the author saw Nicola and Jacopo resting on the pavement late at night. When they author asked Nicola why they were out so late, he said that they were waiting for the last bus from Padua to sell all their papers. The author asked if they should always work that hard because they looked tired, but Nicola said that they were not complaining. The next morning when the author went to them to get his shoes shined, he said to Nicola that the way he had seen Nicola and Jacopo work, they must have earned quite some money. The author also observed that they did not spend money on clothes and had a simple diet for their meals. He asked Nicola what he did with his money. After hearing what the author had said, he turned uneasy and coloured deeply under his sunburn and grew pale.