Why do you think Prince Prospero confined black and red to one room?
Since this "pestilence" that sweeps the kingdom is accompanied by death, both colors represent Death, and they are placed in the seventh room, which is symbolic of man's final stage.
The seven rooms are perceived as the "seven stages of man," with the final room containing the large clock that tolls the hours until midnight, the metaphoric final hour. In the particular case of the "pestilence" that is so deadly, the body bleeds profusely and the skin is covered with "scarlet stains," a bright red rash. Therefore, red is paired with black in the room of Death. When the Red Death intrudes upon the revelry, he is both black and red.
Amidst the metonymy of gloom and horror in the castellated abbey, the revelers and Prince Prospero imagine themselves sealed off from danger because of the fortifications around them. Nevertheless, fate befalls them as the interloper, the Red Death, appears as the clock strikes the twelfth hour. He is "shrouded from head to foot in the habiliments of the grave" and his mask resembles the face of a "stiffened corpse" so well that the guests wonder if it is real.
His vesture is dabbled in blood...and his features...besprinkled with the scarlet horror.
Infuriated by this intrusion of one who mocks that which all the guests fear, Prince Prospero advances through the rooms in rage, but the terrorized guests do not follow. Finally, Prospero enters the seventh room that is black and red. There he utters a "sharp cry" and the dagger meant for the intrusive guest falls to the ground as Prince Prospero himself falls "prostrate in death"; then, one by one the revelers succumb to the Red Death, as well.