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You're the light, you're the night
You're the color of my blood
You're the cure, you're the pain
These three lines are saying that the person that the speaker loves is both the good and bad in her life. For example the speaker says that they're the cure and the pain. They are both the greatest part of the speakers life. Also they are the part of her life that make her depressed and sad. The second line is saying that the person they love is the color of there blood so they are her her heart that is red. And it is also a metaphor, because they are comparing the person and color of blood.