In "The Secret Life of Bees," what made some of the honey purple?

In Sue Monk Kidd's "The Secret Life of Bees," the reader learns the cause of purple honey during an exchange between Lilly and Zach. As Lilly is learning the ins and outs of beekeeping, Zach shows her a beehive which drips with an unusually colored honey: "The frame leaked with honey the color of plums," Kidd writes. In the passage, Zach goes on to explain to Lilly: "When the weather turns dry and the flowers dry up, the bees start sucking elderberry. It makes a purple honey.  People will pay two dollars a jar for purple honey."

This passage, though brief, informs us of two significant factors that go into the making of purple honey.  First, the bees must drink the nectar of a purple-colored plant (elderberry). And secondly, Zach's words imply that this only happens when the bees are somewhat desperate, due to drought conditions and the local flowers drying up.  

Although there is only a brief reference to purple honey in the novel, it appeals to the reader's imagination because it is an unusual color we do not generally associate with honey.  It is an intriguing detail Kidd supplies us with, which is easy for the reader to visualize. Lilly's character echoes the excitement we feel when she exclaim's, "Purple honey!" in amazement.