Indigo

This prompt took me a long time to figure out but then I stumbled across these beauties, called the Lactarious Indigo.

Caterpillar: By the way, I have a few more helpful hints. One side will make you grow taller...

Alice: One side of what?

Caterpillar: ...and the other side will make you grow shorter.

Alice: The other side of what?

Caterpillar: THE MUSHROOM, OF COURSE!