An Aura are the colors around a person that reflect their mood and overall character. Red and orange are "linked to love and baser instincts", yellow is tied to antalytical, purple is passion or spirituality, and gold or white are usually associated with higher powers and metaphysics.
Spirit users Lissa Dragomir, Adrian Ivashkov, Sonya Karp, and Avery Lazar have all demonstrated this ability.
Throughout the series, it is mentioned that Rose's aura is full of bright colors, but is always spiked with darkness. This is because of her taking spirit from Lissa whenever she loses control. Adrian comments that the darkness grows a little darker 'each day', causing Rose to worry about the future. Rose's aura stays the same but after Last Sacrifice the darkness has disappeared completely from her aura.
Adrian has mentioned many times in the Bloodlines Series that Sydney has a special aura. He says that her aura is yellow, the color for smart, analytical people. However, he also states that her aura has spikes of purple, the color of spirituality and passion. Also, when Adrian touches Sydney, her aura deepens and the purple color increases.
According to Sonya, those who are experiencing a spirit dream will always have their respective auras wrapped in gold.