"Mr. Universe Shirts" were mass-produced t-shirts being sold as band merchandise after concerts for Greg's one-man band, Mr. Universe.
The shirts are black with a red "Mr." on it and a blue "Universe" beneath. It has both red and blue streaks going across.
Rose's Mr. Universe Shirt
One notable such shirt belonged to the late Rose Quartz. It was given to her for free by Greg, to go along with her "purchase" (which was also given to her free of charge by Greg) of his CD "Space Train to the Cosmos", much to the annoyance of his manager, Marty. In order to fit her larger frame, Greg had to search through his van for a size XXL shirt.