The term color line was originally used as a reference to the racial segregation that existed in the United States after the abolition of slavery. An article by Frederick Douglass that was titled "The Color Line"[1] was published in the North American Review in 1881. The phrase gained fame after W. E. B. Du Bois' repeated use of it in his 1903 book The Souls of Black Folk.
The phrase sees current usage as a reference to modern racial discrimination in the United States and legalized segregation after the abolition of slavery and the civil rights movement.