A kit for colouring hair comprising: i) a first container containing a composition comprising an oxidative dye; and ii) a second container containing a composition comprising a per-oxygen generating material, in which the second container comprises sides of height X mm, a first end closed by a bottom of external diameter Y mm and a second end opposite the first, having a opening of internal diameter Z mm in which the ratio of X to Y is less than 1:1 and the internal diameter Z mm differs by 15% or less from the external diameter Y mm and in which the internal diameter Z mm is from 60 to 80 mm.
