This is probably a well known factoid but SON OF FRANKENSTEIN was supposed to be filmed in Technicolor but that idea was scrapped so the film was shot in black and white. I think that's why the Monster is wearing a brown woolen vest; to give him a bit more color rather than just the standard black pants, shirt and jacket. Karloff hated the vest to the point when a large cake was brought in to celebrate his birthday, he made sure to smear a good amount of the cake's icing on the vest, insuring he wouldn't have to wear it the rest of the day.
The color footage raises a question; Jack Pierce said the Monster's original coloring was blue/green to simulate dead skin. So why is Boris colored solid green here?