Colour removers help eliminate artificial pigment from your hair without bleaching. They work by destroying the artificial colour molecules and facilitate the washing out of the colour. Your natural colour will be untouched, although keep in mind that if you've previously used a dye containing developer it may have permanently lightened the hair underneath.
It's important to follow the instructions very carefully to get the best from colour removers - particularly the washing and rinsing stages which, although they may seem excessive, are an important part of the colour removal process.
Always do a strand and sensitivity test before use.