Answer
The answer to the question "He was called Velvet Brueghel due to his mastery in rendering what?" is "Fabrics." Pieter Bruegel the Younger, better known as Velvet Brueghel, was a Flemish Renaissance painter who is best known for his landscapes and peasant scenes. His mastery in rendering fabrics was evident in his use of bright colors and his ability to capture the texture and detail of various types of fabric.