Shichi Henge

七変化 Literally "Seven Transformations"

Refers to a series of rapid costume changes or transformations. A lot of Sentai heroines (especially pink rangers) have been known to do a lot of costume changes in an effort to bewilder an enemy.

The number seven probably comes from the number of colors there are in a rainbow.

1: (Usually written using kana alone) lantana (plant, flower);
2: kabuki dance in which the dancer quickly changes his clothes seven times
From Jisho.org