Symbolism, Imagery, Allegory
Compared to Hell's constant darkness and fiery redness, all the light and colors and general beauty of Purgatory offer welcome relief. This imagery also reflects the goodness of God’s works. Many of the scenes in which light and color are at their most vibrant occur in descriptions either of the heavenly bodies or of natural scenes—both of which were necessarily missing from Hell because it’s underground. Also it's, you know, Hell.
This of course points out a big difference between Purgatory and Hell; Purgatory is located on the surface of the earth and more closely resembles the natural human world than the infinite darkness and pain of Hell. Many of the most beautiful scenes are used as time-telling checkpoints (gleaned information from the sun and stars) or are accompanied with phrases suggesting that God is a sort of artist who designs these beautiful places for a useful purpose as well as for sheer delight. Where the work metaphors serve to illustrate the benefits of man’s labor, these lovely landscape scenes often point out God’s good works. Thanks, God!
In general, light seems to represent spiritual enlightenment. The angels shine too brilliantly to be looked upon, suggesting that their state of grace far surpasses any mortal’s. Interestingly, sunlight somehow seems to determine man’s progress. Neither the penitents nor Dante can continue their journey up the mountain once the sun has set; they must take that time to rest. Something about darkness simply stops forward movement.
It could be that man simply cannot see in the darkness and thus cannot tell where he is going. This line of thinking finds much support in the emphasis placed on the five senses in Purgatory. Penitents are constantly being bombarded by images of repentance, hymns heard on the wind, and even the rough texture of their various punishments. It makes sense, though, because how else is man supposed to enjoy God’s works if not through his senses?