Come on guys, exercise your imaginative centers a bit. Rainbows are bright, colorful, playful, remind you of sunny days out at the park with your friends. It's not a huge stretch.
About as much of a stretch as using a unicorn for "delete", because unicorns don't exist.
I'm getting increasingly concerned that web UIs are trading away usability for design in Apple's wake, forgetting the part about "design is how it works".