The colors of the seven rooms are just too juicy a detail not to mean something, aren't they? The black and blood red room seems so obviously to represent death, shouldn't the other rooms mean something too? A lot of commentators have thought that, and there is something of a general agreement among many of them about the meaning of the rooms.
Supposedly, the suite is an allegory of human life. Each room, in other words, corresponds to a different "stage" of human life, which its color suggests. The first clue that the suite is allegorical is that the rooms are arranged from east to west. East is usually the direction associated with "beginnings," and birth, because the sun rises in the east; west (the direction of the sunset) is associated with endings, and death.
According to this reading, the blue room, which is furthest to the east, represents birth. The color suggests the "unknown" from which a human being comes into the world. The next room is purple, a combination of blue (birth) and red (associated with life, intensity) suggests the beginnings of growth. Green, the next color, suggests the "spring" of life (youth), orange the summer and autumn of life. White, the next color, suggests age – think white hair, and bones. Violet (a combination of purple and blue, or purple and grey) is a shadowy color, and represents darkness and death. And black, obviously, is death. Pretty nifty, huh?
Also, notice how there's no red room? Why's that? You might think of red as a better color than orange for summer/autumn, or as a better color than purple for growth. But our guess is that Poe wanted to save the color red in this story especially for its association with blood, fear, and death. That means it's always goes with black, just like the Red Death and the darkness go together at the end of the story, and red and black go together in the seventh room. If there were a red room, it would confuse the color system and obscure the meaning of "red."
Now another interesting thing about the allegorical reading of the rooms is that it gives an added meaning to other bits of the story. The fact that the revelers don't go into the black room indicates their fear of death (although you don't need to give a meaning to each room to figure that one out). But besides that, remember that the Red Death walks from the blue room to the black room – it walks the course of life, leading from birth to death. Prospero follows that course when he chases it: he runs from the blue room to the black room, where he dies. His followers also rush into the black room to unmask the Red Death, and also die. So the course the characters walk in the story is both literally and metaphorically the course from life to death.