Anthocyanins are the phenolic group that is responsible for the color in red wine. They are also antioxidants. More specifically, anthocyanins are one class of flavonoid compounds found in wine. Their color ranges from red to blue.
The color of anthocyanins is pH dependent, with a lower pH having more potential red color visible. Anthocyanins will bind with tannins (or polymerize) to stabilize the color. The presence of sulfur dioxide can inhibit the copolymerization process between the anthocyanins and the tannins by binding directly to the tannins itself. This is termed sulfite bleaching, and is generally avoided in red wine production.