The present invention relates to an oxidative hair colouring and bleaching composition comprising an oxidizing agent, a source of carbonate ions, an alkalising agent, and preformed dyes, utilised at pH 9.5 and below which improved colour delivery, intensity and vibrancy, which are compatible with current dyes and dye precursor systems. Moreover, the compositions of the present invention also exhibit low odour and deliver a high level of lift and lightening equal to the currently utilised ammonia/peroxide systems, whilst reducing the concentration of peroxide and reducing the hair fibre damage.