After World War II, the United States and the Soviet Union found themselves on opposite sides of a "Cold War," and the threat posed by communists in the United States became known as the Red Fear.
For this reason, the government of the United States responded by creating the Committee of Un-American Activities of the House of Representatives, (HUAC).
The HUAC often pressured witnesses to give names and other information that could lead to the arrest of communists and supporters of communists.
The committee members rated the witnesses as "red" if they refused to comply or hesitated to answer the committee's questions.