Katy Perry understood the assignment ... When she appeared on stage, Perry appeared in a simple dress, but once she pulled a ribbon at the back, she spun out the gorgeous blue gown.

Katy Perry dazzled viewers on Sunday night during her second-to-last episode as a judge on American Idol, pulling off a show-stopping Cinderella gown transformation that left fans in awe.

Katy Perry pulled out ... She styled her hair down in a wet grunge look. At one point, the Grammy winner turned around to display a QR code tattoo on her lower back, which, when followed, prompts ...