Mug magic

OK, I really don't understand the science of this mug, but it completely fascinates me. When the mug is empty (or when it has a cold liquid in it), it's a black mug with white letters that read COLD. But if you fill it with a hot liquid, the mug turns white (???!?), and the letters, which say HOT, are black.

As the level of your hot liquid goes down, the top turns back to its original black.

Like I said, I don't get it, but I want to get one. They go for $25 from Charles & Marie.