Colour display tube with a shadow mask, a shield of a ferromagnetic material and a demagnetizing device having a set of demagnetizing coils. The demagnetizing device is provided with means for energizing the demagnetizing coils in two steps which are shifted in time with respect to each other, while the location of the turns of the demagnetizing coils is such that upon the passage of current through only the one coil, the magnetic field generated by said coil is capable of magnetizing those parts of colour selection electrode and screen which are not magnetized upon the passage of current through only the other coil, and conversely.