The Girl with the Dragon Tattoo. The Girl Who Played with Fire. The Girl Who Kicked the Hornet's Nest.
Graceling. Fire. Bitterblue.
The Hunger Games. Catching Fire. Mockingjay.
Notice a pattern? (Or two or three?)
Fire keeps ending up in the second titles of trilogies, and it makes sense that it does. To me, at least, a "fire" title says that the first book may have started things off, but now, now, they're going to get intense. Cool-colored covers are for conclusions. The middle of the story is red-hot, or at least, it should be if it's to attract readers and keep the trilogy going.
Still, the trend makes me glad The Giving Tree never became a trilogy.