This is one of the three candles that are always on the altar in our tradition. (The other two are the Maiden Candle, and the Lord Candle.) This candle is sacred to the Lady, the Goddess of our religion. Traditionally, it's on the Eastern side of the altar. The HPS (High Priestess) usually stands on this side, too, as the representative of the Goddess. Lots of people use another symbol of the Lady on the altar, as well. This can range from a seashell to an elaborate statue, handmade just for the covenstead. I didn't put one in this picture, because there wasn't room for it. But we do use one on our real altar. In fact, we tend to use a different one for every holiday!
This candle, like all the others, may be different colors depending on what is being celebrated. But when in doubt, go with white; it's a good all-purpose color, because it contains all the other colors within it.