The present invention is related to an oxidative colouring composition for keratin fibres especially human hair. The present invention is a composition comprising at least one hair dye and at least one magnesium salt. Furthermore, it is the use of at least one magnesium salt in a composition comprising at least one hair dye for preventing warming up and increase of its volume upon mixing with an aqueous composition comprising at least one oxidizing agent. Present invention is at the same time on the process for colouring keratin fibres with the use of the compositions.
