The narrator describes Prospero's residence thus:
But first let me tell of the rooms in which it was held. There were seven -- an imperial suite. In many palaces, however, such suites form a long and straight vista, while the folding doors slide back nearly to the walls on either hand, so that the view of the whole extent is scarcely impeded. Here the case was very different; as might have been expected from the duke's love of the bizarre. The apartments were so irregularly disposed that the vision embraced but little more than one at a time. There was a sharp turn at every twenty or thirty yards, and at each turn a novel effect. To the right and left, in the middle of each wall, a tall and narrow Gothic window looked out upon a closed corridor which pursued the windings of the suite. These windows were of stained glass whose color varied in accordance with the prevailing hue of the decorations of the chamber into which it opened. That at the eastern extremity was hung, for example, in blue -- and vividly blue were its windows. The second chamber was purple in its ornaments and tapestries, and here the panes were purple. The third was green throughout, and so were the casements. The fourth was furnished and lighted with orange -- the fifth with white -- the sixth with violet. The seventh apartment was closely shrouded in black velvet tapestries that hung all over the ceiling and down the walls, falling in heavy folds upon a carpet of the same material and hue. But in this chamber only, the color of the windows failed to correspond with the decorations. The panes here were scarlet -- a deep blood color. Now in no one of the seven apartments was there any lamp or candelabrum, amid the profusion of golden ornaments that lay scattered to and fro or depended from the roof. There was no light of any kind emanating from lamp or candle within the suite of chambers. But in the corridors that followed the suite, there stood, opposite to each window, a heavy tripod, bearing a brazier of fire that protected its rays through the tinted glass and so glaringly illumined the room. And thus were produced a multitude of gaudy and fantastic appearances. But in the western or black chamber the effect of the fire-light that streamed upon the dark hangings through the blood-tinted panes, was ghastly in the extreme, and produced so wild a look upon the countenances of those who entered, that there were few of the company bold enough to set foot within its precincts at all.
The key points to remember are as follows:
1. There were seven rooms in all.
2. The rooms were structured in a bizarre manner so that "the vision embraced but little more than one at a time."
3. All the rooms were of a different color: blue, purple, green, orange, white, violet and black.
4. The scheme of lighting for each of these rooms was even more bizarre especially for the last room, so much so that no one wanted to enter this room.
Prospero’s suite consists of seven rooms, a number which often figures in numerology (seven deadly sins, seven champions of Christendom, seven as the first roll of the dice, and so on). The rooms are, successively, blue, purple, green, orange, white, violet, and black with scarlet ("a deep blood color" [paragraph 4]). Light comes through tinted windows from fires blazing in braziers supported by tripods, so that the light in the green room is tinted green, and so on. Poe’s intention here is to evoke a mood of eeriness and unreality, except, perhaps, for the white light in the fifth room, for the room lights reduce everything to the same color, and the revelers change colors as they go back and forth from room to room.