Langston Hughes wrote during the Harlem Renaissance period (1920s). Given his work extended past the 1920s, he has been associated with both the Modern and Post-Colonial periods as well.
That said, a realistic play (like Hughes' Mullato:A Play of the Deep South) needs to adhere to how things really were. Realists, the writers who emerged after the Romantics, desired to show life as it was. Realists created common characters, living in common places, dealing with common problems. Although not a Realist, Hughes' play most certainly contains characteristics typical of the Realist text.
The play depicts the life of a plantation owner, Colonel Thomas Norwood and his brood of mulatto children (mothered by his housekeeper Cora). The setting of the play takes place on Norwood's plantation (a typical scene for the South). That said, the dilemma that Norwood finds himself in (the idea of marrying a white woman instead of continually sleeping with Cora). The idea of how others would look at Norwood, if he does not marry, is very important to him.
That said, without the realistic issues presented in the play, it would not speak to the actual customs and ideologies of the time. Hughes would, in essence, lose his influence and impact on African American literature. Given the racism and discrimination seen in the play, the setting of Mulatto is of the utmost importance (regarding realism).