Many scholars believe that the seven rooms in Prince Prospero's castle refer to a speech made by a character named Jacques, from Shakespeare's As You Like It. In this speech, Jacques addresses what he calls the "seven ages" of a person's life, the seven parts or roles that he plays:
All the world's a stage,
And all the men and women merely players;
They have their exits and their entrances,
And one man in his time plays many parts,
His acts being seven ages. (2.7.139-143)
The interpretation of the rooms as symbolic of a person's path from infancy to death, ending with a black clock -- both the color and the object often associated with mortality and death's inevitability as our time ticks away -- is reinforced by the way the rooms are situated, from east to west. The sun rises in the east, the birth of day (often symbolic of birth in general), and sets in the west, the death of day (often symbolic of death in general).
In this scheme, the blue room represents the unknown from which we come, as infants. Purple, then, the second room, combines blue (unknown) and red (which symbolizes life and intensity), signifying the beginning of our growth. The green room, third, represents youth and vitality, as green so often does. Orange, the fourth room, represents the summer and fall of life (our prime and early decline. White, then, the fifth room, is old age (as our hair grows white with age). Violet, in the sixth room, combines blue and purple, suggesting shadow and decline, even a return to the unknown from which we came. Finally, the seventh room of black and red symbolizes death, and this is why the revelers stay out of that room and dread the chimes of the clock, especially at midnight (the death of day); they fear death.