Souls play an important role in the Soul Eater Universe. A Soul, along with it's human carrier, can stray from goodness and easily choose a life of evil. Evil beings, namely Kishin, happen to eat other Souls in order to gain more power, proving how low one could go for Souls in the storyline.
Soul Eater Evans, about to devour a Kishin Egg
Color Coding
Every Soul is a certain color by the goodness or taint of justice, and is color-coded to identify it right away.
Canon
Light Blue
If a Soul is light blue, it means that the person is pure and is quite frankly innocent, free of an evil grasp. These Soul are usually eaten by evil beings like Kishin to increase their power.
Deep Red
If a Soul is deep red, it means that the person is quite evil and could possibly be a Kishin through all the evil doings committed. Also known as Kishin Eggs, these Souls are usually eaten by Weapons, given permission from their Meisters in order to make them more powerful.
Violet/Purple
If a Soul is purple or violet, it means that the person is a Witch, not one of pure evil, but mischief equally. These Souls are, also, eaten by Weapons, as they are an equivalent to one hundred Kishin Eggs.
Fanon
Green
If a Soul is green, it means that the person is a Shinigami, or God of Death (akin to the Grim Reaper)
Medusa's large-scale Soul, as seen by Maka
Viewing a Soul
Shinigami's gigantic Soul protecting Death City
Some Meisters have the power to view a Soul, shown surrounding the person being observed. Souls of others can come in many different forms, depending on the personality and the amount of power. For instance, Medusa's soul is about one sixteenth of Shibusen, is violet, and has many protruding serpent-like arrows, while Shinigami's soul, which is yellow-green in color, coats all of Death City, acting as a sort of huge forcefield.
Soul Wavelengths and Soul Resonance
Two Souls in the midst of a Soul Resonance
A Soul Wavelength is the beat of one's Soul and is something powerful that is capable of enhancing an attack if needed. Meisters and Weapons use this to their advantage and combine their Soul Wavelengths to create a huge enhancement to an attack with their Weaponry.
Death Scythe Goal
To become a true Death Scythe, it is required that he or she eat ninety-nine Kishin Eggs and one Witch Soul, in which case doesn't equal 199 souls, but 100, according to Shinigami.