Depending on the shade chosen, a permanent hair colour can be applied over a tone-on-tone hair colour such as Casting Crème Gloss providing it has been close to 28 shampoos since you coloured your hair with Casting Crème Gloss. Otherwise, your hair will have build-up of hair dye and the result will be darker than expected. Casting Crème Gloss can be applied over an existing shade to refresh the colour. It is best to select a shade that matches the existing colour or one that is one to two shades darker. A lighter shade will potentially not be as visible as you would desive, so please keep this in mind when selecting your shade. For assistance, please call our team of experienced hairdressers in the L'Oréal Paris Advisory Bureau on 1300 659 359, or alternatively email your enquiry to us at abureau@au.loreal.com. They are happy to help you with any of your hair colour queries.