In “Back to the Future,” Marty’s Calvin Klein underwear was purple.
The color is revealed after Marty McFly arrives in 1955 and is discovered by his future mother, Lorraine Baines. She sees “Calvin Klein” printed on his underwear and assumes that Calvin is his first name. When Marty explains that Calvin Klein is a clothing brand, the misunderstanding becomes one of the film’s most memorable jokes.
The scene helps establish the movie’s central contrast between Marty’s modern 1985 world and Lorraine’s perspective in 1955. To Lorraine, Marty’s branded, brightly colored underwear seems unusually strange and fashionable. The joke also plays on Calvin Klein’s growing cultural visibility as an American fashion label during the 1980s.
A common mix-up is recalling the underwear as red, partly because some early screenplay versions and printed script reproductions use different wording. In the released 1985 film, however, Lorraine explicitly refers to Marty’s underwear as purple. Marty’s “Calvin” nickname is therefore based on the label, not his actual name.